To add an extra charge when a customer uses a credit card to pay for an order:
From the OMS click Settings / Edit Compatible Software / Payment Gateways / Preferences / Authorization Amount. Select 'Order Total plus X percentage'. This will authorize the order total plus a percentage. (We recommend reading the Authorization Amount Tool Tip thoroughly before making this change.)
Please note that to capture this amount in addition to the order total you will need to take an extra processing step:
When capturing funds on orders, you will not be able to batch process or simply click on the $ tag icon on the order list page in your OMS, because the capture will be higher than the order total. Instead, on a per order basis, you will need to click through to the Credit Card Terminal from the order summary page (blue button at the bottom), and click Prior Auth Capture.
This will open a window that will reflect the default to charge the order total. You must change that selection to 'Other Amount' and enter the amount you wish to capture (the authorized amount, if the order hasn't changed).
For orders that were in the system before you updated your authorization amount, you may need to void the initial authorization and auth/capture for the increased amount, because you won't be able to capture more than the authorized amount.