With the Donations app, you have the option to display a checkbox at the bottom of the donation form that, if checked by the visitor, will add a percentage you specify to their donation. Details of the percentage and nominal amounts are saved to Soapbox where you can view them when exporting donations from Donations > Transactions. You can also save them to Salesforce.
To have details of the transaction fee saved to Salesforce, you must manually create the following fields on both the Lead and Opportunity objects and update the Lead mapping accordingly. When defining the API Name, enter only the string proceeding __c. Salesforce automatically adds __c to custom fields:
Field Label: Transaction Fee Amount Covered
API Name: sbx_TransactionFeeAmountCovered__c
Data Type: Currency (16, 2)
Field Label: Transaction Percent Amount Covered
API Name: sbx_TransactionFeePercentCovered__c
Data Type: Percent (16, 2)