Just to be clear, I didn't mean verification on our end -- I meant on yours. If the customer wants to look through their payment history they will not be able to distinguish contracts apart from each other.
Important is that can be verified that ALL the earned bitcoins during the week are sent from you and all the money are arrived at the customer. It is not important to know which part of them come for one contract or another. In this point of view (for all the earned bitcoins) if there it will be problems it is indispensable that they can be found and verified by everyone. In this way everything will be clear for both of parties.