What is a fixed convenience fee?

Excuse me, could you please explain what exactly is meant by a "fixed convenience fee"? I've come across this term in the context of cryptocurrency transactions and financial services, but I'm not entirely clear on how it differs from other types of fees or charges. Is it a standard, pre-determined amount that's added to transactions regardless of the amount being exchanged? Or does it vary based on certain criteria? I'd appreciate any clarification you can provide.