diff --git a/README.md b/README.md
index 080ccfd..2ad8da8 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,18 @@ specific to this project.
   via an httpOnly session cookie this app mints itself; see "Orders &
   customer accounts" below.
 
+## Shared modules
+
+`@einfach-produktiv/invoicing` (a separate repo, consumed as a git
+dependency — see "Invoice PDFs" below) is where framework-agnostic logic
+shared with the Payload backend lives, so it's written once instead of
+maintained as two independently-drifting copies. Beyond invoice/e-invoice
+PDF generation and tax-breakdown math, it also holds VIES VAT-ID checking,
+VAT-ID format validation, PLZ digit-count validation, and shipping-carrier
+tracking-label logic — all previously hand-duplicated `app/lib/*.ts` files
+in this repo, now imported from the shared package instead. See that
+package's own README for what's in it and why each piece was unified.
+
 ## Getting started
 
 ```bash