The form submits via GET to /sessions/checkout with query params: ?classic=2&cheese=1&double=0 (item id = quantity). The server parses this and passes basket items to the checkout page.
When the Drop-in loads, it calls POST /api/sessions/create with { items: [{ id, quantity }] } to create a Yeti session. Amounts are in minor units (pence).
For the full Sessions flow, see the Sessions docs and the broader Yeti ECOM payments overview.
