It is a joke. Originates from a literal meaning of paying someone for a job. Some years back, pre electronic payments or wire, we would still write a cheque and post it to the person as a form of a payment. They would usually get a confirmation of a cheque being posted = your cheque is in the post. He is thanking for the endorsement and making a funny remark that he should get paid for it by saying “ cheque is in the post” P

Hi Jessica, In the US we would say, "The check (or, 'your check') is in the mail."