Scan a tag
Every unit carries a tag with a chip and a QR code. Scanning it opens that unit's page: check-out, the site taps, the return.
On an iPhone, don't use this page
Hold the top edge of the phone to the chip with the screen on and nothing open. A banner drops down and the unit page opens by itself — no app, no permission, no button. That is iPhone XR and newer; on an iPhone 7, 8 or X add the NFC Tag Reader button to Control Centre once and tap it first.
Apple gives a web page no chip reader at all, so no button on this site can start one on an iPhone. The chip still works — the phone reads it, not the page.
On this page
Chrome on Android reads the chip from the button below. Every other phone and every computer reads the QR code on the same label with the camera, which the button switches to by itself. Both are printed on every tag.
Or type the code
On an iPhone XR or newer, unlock the phone and hold its top edge to the chip with nothing open: a banner drops down and the unit page opens. On an iPhone 7, 8 or X add the NFC Tag Reader button to Control Centre once. Safari cannot open the chip reader from inside a web page, so the button above uses the camera on the QR code instead — every label carries both.