I build my own carts, the flow is what you can expect from a normal app: users see the item, use a form to add a quantity of that item to their cart. When they’re done with choosing items, they navigate to the cart, press a button to record that full order.
The only struggle is with the automatic confirmation of payment, as of now I don’t think there’s a good way to do that automatically but I think @Wiz.Wazeer is working on it.