If a customers credit card is saved for a reservation it can be charged like this.

1. In reservation view click the 'Charge card' icon that will appear only if there is a credit card saved for this reservation.

2. Clicking it will open payment view so you can select which part of the reservation you want to charge for.

The 'Charge token' dropdown shows which credit card that will be charged, if there are more then one credit card saved in the reservation  they will appear as options in this dropdown. Click 'Save' to charge the card.