Let's look at what they have to do to earn that transfer fee:
send in the FFL paperwork to the online dealer you purchased from
receive the delivery, log in their book
contact you, do the 4473 paperwork, conduct background check
transfer the gun to you, log out the book
keep the 4473 on file permanently
Online dealer only need to pack and ship like selling a pair of socks, just saying.
Some of the long-time dealers here don’t have to send in the FFL paperwork if it’s already on file.
And at least one does not contact you when your firearm is in - it’s up to you to track it. But yeah, for $10 that’s fine