So basically you used a service that costs money, you didn't pay any, and now you're peeved that they'd actually like to collect?

> They could have dropped any other amount they wanted

They probably told you "the usual amount". Are there any grounds for suspicion that they are quoting you some outrageous price?

I don't see what could qualify as "shady" here.

And btw: from a very possible point of view they might think you didn't forget to read the small print, you deliberately tried to defraud them by acting as if you were non-commercial.

They can't look inside your head.