This article explains the option a Donor is given to increase the amount given when choosing to make a donation with a Credit Card.
The donor will see this option in the payment method step of donating if they choose to use a credit card. When the Donor selects the box to increase their donation to help offset the processing fees, the additional amount they have chosen to donate will be seen immediately as the Processing Fee Donation along with their new total.
*To see donations using this offset option, run and download the CSV version of the Giving Detail report.
Check this out for more information About Credit Card Offset Processing Costs.