By default, the campaign will give your supporters the option to cover the processing fee for the transaction. It will be displayed as a box with an Edit link:
You can change the Edit link to a checkbox that can be unchecked by supporters, or you can also choose to absorb the cost of the processing fee by eliminating the option to cover the processing fee for your supporters.
To change the Processing Fee Settings, go to the Campaigns > Edit This Campaign > Campaign Settings > Processing Fee Settings.
Here, you can choose how the option is displayed to supporters (with an Edit link or a checkbox to make changes), or you can choose to have your organization cover all the processing fees. (Please note that if your organization opts to cover the processing fees, these transactions will not qualify for the Rate Guarantee.)
Hit Save when you're done!