diff --git a/conservancy_beancount/plugin/__init__.py b/conservancy_beancount/plugin/__init__.py
index 7c7f368..cc83211 100644
--- a/conservancy_beancount/plugin/__init__.py
+++ b/conservancy_beancount/plugin/__init__.py
@@ -51,6 +51,7 @@ class HookRegistry:
         '.meta_expense_allocation': None,
         '.meta_income_type': None,
         '.meta_invoice': None,
+        '.meta_paypal_id': ['MetaPayPalID'],
         '.meta_project': None,
         '.meta_receipt': None,
         '.meta_receivable_documentation': None,
diff --git a/conservancy_beancount/plugin/meta_paypal_id.py b/conservancy_beancount/plugin/meta_paypal_id.py
new file mode 100644
index 0000000..cf595f5
--- /dev/null
+++ b/conservancy_beancount/plugin/meta_paypal_id.py
@@ -0,0 +1,52 @@
+"""meta_paypal_id - Validate paypal-id metadata"""
+# Copyright © 2020  Brett Smith
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+import re
+
+from . import core
+from .. import data
+from .. import errors as errormod
+from ..beancount_types import (
+    Transaction,
+)
+
+class MetaPayPalID(core._PostingHook):
+    METADATA_KEY = 'paypal-id'
+    HOOK_GROUPS = frozenset(['metadata', METADATA_KEY])
+    TXN_ID_RE = re.compile(r'^[A-Z0-9]{17}$')
+    INVOICE_ID_RE = re.compile(r'^INV2(?:-[A-Z0-9]{4}){4}$')
+
+    def _run_on_post(self, txn: Transaction, post: data.Posting) -> bool:
+        if post.account.is_under('Assets:PayPal'):
+            return True
+        elif post.account.is_under('Assets:Receivable'):
+            return self.METADATA_KEY in post.meta
+        else:
+            return False
+
+    def post_run(self, txn: Transaction, post: data.Posting) -> errormod.Iter:
+        if post.account.is_under('Assets:Receivable'):
+            regexp = self.INVOICE_ID_RE
+        else:
+            regexp = self.TXN_ID_RE
+        value = post.meta.get(self.METADATA_KEY)
+        try:
+            # A bad argument type is okay because we catch the TypeError.
+            match = regexp.match(value)  # type:ignore[arg-type]
+        except TypeError:
+            match = None
+        if match is None:
+            yield errormod.InvalidMetadataError(txn, self.METADATA_KEY, value, post)
diff --git a/tests/test_meta_paypal_id.py b/tests/test_meta_paypal_id.py
new file mode 100644
index 0000000..c70d99a
--- /dev/null
+++ b/tests/test_meta_paypal_id.py
@@ -0,0 +1,189 @@
+"""Test validation of paypal-id metadata"""
+# Copyright © 2020  Brett Smith
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+import itertools
+
+import pytest
+
+from . import testutil
+
+from conservancy_beancount.plugin import meta_paypal_id
+
+VALID_TXN_IDS = {
+    # Basically normal ids.
+    'A1234567890BCDEFG',
+    '12345HIJKLNMO6780',
+    # All numeric
+    '05678901234567890',
+    # All alphabetic
+    'CDEFHSNOECRUHOECR',
+}
+
+VALID_INVOICE_IDS = {
+    'INV2-ABCD-1234-EFGH-6789',
+    # All numeric
+    'INV2-1010-2020-3030-4567',
+    # All alphabetic
+    'INV2-ABCD-EFGH-IJKL-MNOP',
+}
+
+VALID_VALUES = VALID_TXN_IDS | VALID_INVOICE_IDS
+
+INVALID_VALUES = {
+    # Empty
+    '',
+    ' ',
+    # Punctuation and whitespace
+    'Z12345-67890QRSTU',
+    'Y12345.67890QRSTU',
+    'X12345_67890QRSTU',
+    'W12345 67890QRSTU',
+    'INV2-ABCD.1234-EFGH-7890',
+    'INV2-ABCD-1234_EFGH-7890',
+    'INV2-ABCD-1234-EFGH 7890',
+    # Too short
+    'Q1234567890RSTUV',
+    'INV2-ABCD-1234-EFGH-789',
+    # Too long
+    'V123456789012345WX',
+    'INV2-ABCD-1234-EFGH-78900',
+    'INV2-ABCD-1234-EFGH-7890-IJKL',
+    # Bad cadence
+    'INV2-ABCD-1234-EFG-H7890',
+    'INV2ABCD-123-EFG-456-789',
+    'INV2ABCDEFGHIJKLMNOPQRST',
+}
+
+ACCOUNTS = itertools.cycle([
+    'Assets:PayPal',
+    'Assets:Receivable:Accounts',
+])
+
+TEST_KEY = 'paypal-id'
+INVALID_MSG = f"{{}} has invalid {TEST_KEY}: {{}}".format
+BAD_TYPE_MSG = f"{{}} has wrong type of {TEST_KEY}: expected str but is a {{}}".format
+
+@pytest.fixture(scope='module')
+def hook():
+    config = testutil.TestConfig()
+    return meta_paypal_id.MetaPayPalID(config)
+
+def paypal_account_for_id(paypal_id):
+    if paypal_id.startswith('INV'):
+        return 'Assets:Receivable:Accounts'
+    else:
+        return 'Assets:PayPal'
+
+@pytest.mark.parametrize('src_value', VALID_VALUES)
+def test_valid_values_on_postings(hook, src_value):
+    txn = testutil.Transaction(postings=[
+        ('Income:Donations', -25),
+        (paypal_account_for_id(src_value), 25, {TEST_KEY: src_value}),
+    ])
+    assert not list(hook.run(txn))
+
+@pytest.mark.parametrize('src_value', INVALID_VALUES)
+def test_invalid_values_on_postings(hook, src_value):
+    acct = paypal_account_for_id(src_value)
+    txn = testutil.Transaction(postings=[
+        ('Income:Donations', -25),
+        (acct, 25, {TEST_KEY: src_value}),
+    ])
+    actual = {error.message for error in hook.run(txn)}
+    assert actual == {INVALID_MSG(acct, src_value)}
+
+@pytest.mark.parametrize('src_value,acct', testutil.combine_values(
+    testutil.NON_STRING_METADATA_VALUES,
+    ACCOUNTS,
+))
+def test_bad_type_values_on_postings(hook, src_value, acct):
+    txn = testutil.Transaction(postings=[
+        ('Income:Donations', -25),
+        (acct, 25, {TEST_KEY: src_value}),
+    ])
+    actual = {error.message for error in hook.run(txn)}
+    assert actual == {BAD_TYPE_MSG(acct, type(src_value).__name__)}
+
+@pytest.mark.parametrize('src_value', VALID_VALUES)
+def test_valid_values_on_transactions(hook, src_value):
+    txn = testutil.Transaction(**{TEST_KEY: src_value}, postings=[
+        ('Income:Donations', -25),
+        (paypal_account_for_id(src_value), 25),
+    ])
+    assert not list(hook.run(txn))
+
+@pytest.mark.parametrize('src_value', INVALID_VALUES)
+def test_invalid_values_on_transactions(hook, src_value):
+    acct = paypal_account_for_id(src_value)
+    txn = testutil.Transaction(**{TEST_KEY: src_value}, postings=[
+        ('Income:Donations', -25),
+        (acct, 25),
+    ])
+    actual = {error.message for error in hook.run(txn)}
+    assert actual == {INVALID_MSG(acct, src_value)}
+
+@pytest.mark.parametrize('src_value,acct', testutil.combine_values(
+    testutil.NON_STRING_METADATA_VALUES,
+    ACCOUNTS,
+))
+def test_bad_type_values_on_transactions(hook, src_value, acct):
+    txn = testutil.Transaction(**{TEST_KEY: src_value}, postings=[
+        ('Income:Donations', -25),
+        (acct, 25),
+    ])
+    actual = {error.message for error in hook.run(txn)}
+    assert actual == {BAD_TYPE_MSG(acct, type(src_value).__name__)}
+
+@pytest.mark.parametrize('src_value', VALID_INVOICE_IDS)
+def test_invoice_ids_not_accepted_for_non_accruals(hook, src_value):
+    acct = 'Assets:PayPal'
+    txn = testutil.Transaction(**{TEST_KEY: src_value}, postings=[
+        ('Income:Donations', -25),
+        (acct, 25),
+    ])
+    actual = {error.message for error in hook.run(txn)}
+    assert actual == {INVALID_MSG(acct, src_value)}
+
+@pytest.mark.parametrize('src_value', VALID_TXN_IDS)
+def test_transaction_ids_not_accepted_for_accruals(hook, src_value):
+    acct = 'Assets:Receivable:Accounts'
+    txn = testutil.Transaction(**{TEST_KEY: src_value}, postings=[
+        ('Income:Donations', -25),
+        (acct, 25),
+    ])
+    actual = {error.message for error in hook.run(txn)}
+    assert actual == {INVALID_MSG(acct, src_value)}
+
+def test_required_for_assets_paypal(hook):
+    acct = 'Assets:PayPal'
+    txn = testutil.Transaction(postings=[
+        ('Income:Donations', -35),
+        (acct, 35),
+    ])
+    actual = {error.message for error in hook.run(txn)}
+    assert actual == {f"{acct} missing {TEST_KEY}"}
+
+@pytest.mark.parametrize('txn_id,inv_id', testutil.combine_values(
+    VALID_TXN_IDS,
+    VALID_INVOICE_IDS,
+))
+def test_invoice_payment_transaction_ok(hook, txn_id, inv_id):
+    txn = testutil.Transaction(**{TEST_KEY: txn_id}, postings=[
+        ('Assets:Receivable:Accounts', -100, {TEST_KEY: inv_id}),
+        ('Assets:PayPal', 97),
+        ('Expenses:BankingFees', 3),
+    ])
+    assert not list(hook.run(txn))