|Can they do this? Yeah, they already did, and it cost you $15 to learn what kind of company you were dealing with.|
The real question is whether you will allow them to do this to you in the future. If it's not in writing, it's not part of the contract you made with them. No one gets to add requirements after the fact. IF they told you during the scheduling call that faxbacks were required, but didn't include that in the written confirmation, and you remembered what they said and sent the faxbacks anyway, I'm pretty sure they would have found another way to ding you for the $15 - because they only ever wanted to pay you $85 rather than the $100 you quoted and they accepted.
Sleaze-bag companies like these need to be outed for the benefit of others in the notary community, and kicked to the curb immediately - no second chances unless you really want to lose another $15 finding out whether they will actually honor the price you quoted and they agreed to. That part is up to you.