What happens at pickup
Once a time is agreed, the rest of the order plays out in a fixed sequence: the seller gets it ready, you collect it, and each side confirms their half.
The seller marks it ready
When your order is scheduled, the seller's next move is to mark it ready for pickup. This is also the moment your card is actually charged, unless you confirm pickup first, in which case your confirm is the charge. If something still needs weighing, the charge waits until pickup. Before this, the amount was only held on your card, except the deposit on an order for a date far ahead, which was charged at checkout. If the seller has written pickup instructions for this stage (a gate code, which cooler, the name on the bag), they arrive as a pinned note in the order chat rather than an ordinary message, so it's still there if you scroll back an hour later standing in the driveway.
You confirm the pickup
Once you have collected everything, open the order and tap Mark as picked up. That confirmation is what completes the order and pays the seller. If nobody confirms, the order completes on its own a day or two after the agreed pickup time, and the charge is taken then if it was not taken already.
For a delivery order, the equivalent action is Confirm delivery received, since there's no seller-side "ready" step to hand something over in person.
If you never show up
If a pickup goes well past its scheduled time, the seller gets an option to report that you never came. Doing so cancels the order, and the payment is kept: what was held on your card is charged, or on an order for a date far ahead, the deposit and any balance on hold. The seller can still choose to refund it, for up to 7 days after the payment was taken.
After pickup
A completed order gives both sides a chance to leave a review, and gives you the option to report a problem if what you received doesn't match what was listed.

