The Payment Method is a common field in Salesforce Opportunity Object. It is also a Picklist field. Donorbox has the ability export to a picklist field, but there is one important caveat: the field values between Donorbox and Salesforce must match.
The most obvious matching for the SF Payment Method field is the Donorbox's Donation Type Field. You can map it from https://donorbox.org/org/sf_setting
Please note that the the Donorbox's Donation Type field has these possible these values: stripe, ach, paypal_express, cash, check, and credit_card.
For the export to work, the SF Payment Method Picklist field must accept these values as well.