If you're required to business only with entities registered with some 3rd party (who? some government? your government?) then you should be able to refer to them for whatever documentation rules there are.
Maybe your accountant has not sufficiently explained the details you you, or is just outright mistaken themselves?
If someone didn't even bothered to pay for business license, and hire an accountant, I highly doubt they will hire a lawyer.
Note, I'm endorsing not paying people for the services rendered,but this should be explained upfront, that proper invoice is needed in order to get paid.
It is probably smarter to do something better than handwritten invoice but I am glad there are not so many hurdles to starting a business here. I have dealt with a lot of contractors here who don’t even issue a paper invoice for small jobs, they just tell me how much I owe and I just pay.
Small jobs probably are not worth suing for. But I’m sure if I don’t pay for a big enough job, they would sue me.
you need to save a paper or electronic trail, every email or instant message should be treated as a future evidence in court
while technically US citizens don't need a business license to be self-employed, federal or local state agencies may still charge you, as some people were forced to pay for a business license just b/c they were running blogs with ads, and received some pennies in ads revenue