Most of time they will want to see invoices and sometimes expenses. They just need to tick boxes on their side and need paperwork with numbers on it...lol. They could not careless as long as paperwork has address, telephone number, email, what was done and it all looks legit. In worse case they can ask your client to provide a bank statement from their account they used to pay your from. Its invasive but that all part of the banks Know Your Clients Client (KYCC).