We treat it as 100% deductible donation and then we allocate the funds between the fees paid and the donation part. You can treat the amount paid to cover costs as a restricted donation.

Here is another take – https://fundraiseup.com/support/tax-deductible-fees/

fundraiseup.com

Understanding fee coverage and tax deductibility | Fundraise Up Knowledge Base

When supporters make contributions through Fundraise Up, they may have the option to cover the platform and payment processing fees associated with their donation. This feature, known as "Transaction costs...