Nine thousand receipts

The mistake

The checkout's final step calls a server-side print routine that resolves "the printer" from a single global configuration value — the one printer registered when the shop was set up, which is the office machine on the local network. Nothing distinguishes a document meant for the customer's own browser from one meant for shop staff, so every customer's receipt is queued as a job on that one device. Because the print call is awaited inline in the checkout transaction, the queue backing up behind an empty paper tray blocks payment completion for everyone.