Disclaimer: This is just an entertaining notion and not something I or anyone I know of is currently implenting or is planning to implement. Though I'm sure people have been tempted :P
The following question is purely hypothetical and shouldn't be taken seriously.
Would it be illegal to charge "asshole-tax" or would it fall under "service fees"? :P
---- not about asshole-tax ----
On a more serious note, can you charge additional fees if the customer is overdue for paying?
I do believe you have to be registered as a company before you can sic a repo officer on a non-paying customer, and only if you have a contract of sorts. Has that ever happened in the fandom? I've never heard of it but with an increasing amount of "professional" artists you'd think it's inevitable.