When no specific agreement is made between two parties, the halacha is that minhag hamakom prevails. That would apply to both the rate, and the expected date of payment.
Is there a prevailing norm on either that is firmly established? I’m not so certain that there is.
And, even if there is, it *still* makes good sense to have everything specified up front. I *really* find it hard to believe that you think that there is something wrong with specifying fees for services up front.
The Wolf