I'm sorry, but although that is technically correct, it's also absurd.
If you sell an item on Reverb for $1000, then their cut went from $35 to... $50 OMG NOOOO!!! Or, you can simply charge an extra $15 (like a bunch of other sellers will do) to make up the difference.
Externalities like this are downright common in business. If this change results in you losing 43% of your profit margin on each sale, then maybe you're in the wrong business.
I'm not sure how to break this to you, but... that's
how it works. They provide a service; they charge you for that service; sometimes, they donate some of those revenues. Where else are they supposed to get funds for donations?