Donor-covered fees are an option organizations can choose whether or not to activate. Go to Settings > Organization to access this option:
Once you choose 'true' to allow for donor-covered fees (and click 'update organization' to save your changes), a new option will appear on the donation page:
This option allows for donors to check the box saying they'd like to include the tax-deductible processing fee. The donation total will then add the processing fee to their desired donation.
Once a donor chooses to include the processing fee and processes their transaction, the transaction will appear in the transaction list with a '+' sign next to it. If you hover over the +, it will display this text:
This way, admin can see whether or not a donor has chosen to include the processing fees for a particular donation.