Thermal printer receipts fade within months — sometimes weeks in a hot car or a sunny window. If a customer needs that receipt later for a warranty claim or expense report, a faded strip of paper isn't much use. A QR code linking to a digital copy solves this cheaply.
What actually needs a QR, and what doesn't
You don't need to build a customer portal. If you already generate a PDF or an image of each receipt (many POS systems do), host it somewhere accessible — even a shared cloud folder link works — and print a website QR code pointing to that specific file on the receipt itself.
One link per receipt, not one link for everyone
A QR pointing to a general "all our receipts" page is useless and a genuine privacy problem — customers would see each other's purchases. Each receipt's QR code needs to point to that specific transaction's file only.
This isn't a replacement for proper accounting software
For real tax and bookkeeping needs, this is a convenience layer, not a system of record. Use it for "customer wants a backup copy," not as your actual invoice archive.
A simpler alternative
If generating a unique link per receipt isn't practical for your setup, a QR code linking to your business's general contact or support page — "lost your receipt? contact us" — covers most of the same need with far less setup work.