Charge a custom or cart-total amount through Overskill Payments (point-of-sale style checkout)

Let an app charge an arbitrary amount at checkout — a running cart total, a quoted price, or an in-person sale at a counter — through Overskill Payments. Today checkout bills fixed-price catalog items (one-time or subscription plans) that were set up ahead of time; the amount cannot be set per transaction and there is no in-person / tap-to-pay path inside an app.