Distinguish the message from the document.
An order confirmation acknowledges an order. A payment message concerns payment. An invoice records the sale under the invoice setup you have chosen. They can arrive together, but they are not automatically the same object.
WooCommerce extensions can add invoices to selected emails. PDF Invoices & Packing Slips for WooCommerce, for example, is an extension with document and attachment settings. Its presence is separate from WooCommerce core. Check the installed edition and supported correction documents instead of assuming all PDF tools work identically.
Choose one place to issue the invoice.
If the accounting system issues the sales invoice, a second invoice plugin should not independently issue another number for that sale. If the shop issues it, establish how that document and identifier reach the accounts. Agree numbering, company details and the appropriate document requirements with your accountant.
Choose the trigger deliberately. An order can await payment, be paid later or be partially refunded. Generating an invoice at order placement has different consequences from generating it after a chosen payment state. Match the rule to the actual business and payment methods.
Test more than whether a PDF exists.
In this fictional test, order #2041 is paid and the configured invoice system issues INV-2041. The identifier is illustrative, not a numbering recommendation. Follow that same purchase through these checks.
| Step | Expected evidence |
|---|---|
| Order created | The intended trigger has or has not occurred |
| Payment confirmed | One invoice is issued according to the agreed rule |
| Email delivered | The correct customer receives the correct attachment or secure link |
| Customer asks again | The same issued document can be retrieved |
| Partial return | The configured correction refers back to the original sale |
Treat a resend and a correction differently.
Resending an existing document should not silently issue a new invoice number. Changing the sale may instead require a correction through your chosen document process. Check which features are included in your extension or accounting service; do not assume credit notes are part of every free edition.
Use fictional customers for delivery tests and inspect the recipient as well as the filename. If the right document exists but the email is missing, investigate delivery separately. The order-email article explains that distinction. Keep jurisdiction-specific invoice and tax choices with the person responsible for your accounts.