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They obtain an "authorization" in some amount, probably the bill minus tip, then fulfill it later for the amount of the bill plus tip. This is a ubiquitous practice with payment cards - for example, when you pay for gas with one, the POS authorizes some small amount to ensure the account can accept debits before unlocking the pump, and later fulfills the authorization for the price of the fuel you purchased.

Authorizations of this sort count against your available balance, and time out eventually if not fulfilled; when you look at your account and see "pending charges", this is what they actually are. Not all charges use this process - when the total amount is immediately known, it's generally immediately charged as well. But in any case where the amount may change or the account requires validation before a transaction can proceed, this is how it's done.

Presumably, since a valid authorization constitutes an agreement to pay the charge (hence the language under the signature line on your bar tab), the "enable" step you perform via the API would permit the authorization to occur, and fulfillment would work the same way it does now.

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