If you wanted to add on a fixed fee or percentage to the prices you send to a particular OTA, this is easily achieved.
From Settings at the bottom of the menu, click OTA connections.
Click through to the OTA in question, and on the next page you'll see Price Adjustment, and a Type drop down.
On selecting a Type, you can then enter a value in the Price adjustment field.
Finally scroll down and click the Save button at the bottom of this section.