..I think it will make no sense if service providers ask for upfront payment before they can perform their functions to this effect although it will also depend on their terms and if their clients will consent to such agreement if they really exist but ordinarily i think asking for upfront before recovering could trigger suspicion among potential clients, so buddy you are right on all you said, in this kind of function,not is always service before the agreed percentage can be given.
There service relies on if the customer trust them. If the client trust them, any agreement can be done and an upfront payment may be paid subsequently. Because it does not have to do with the clients only, even the service renders can be scammed. If they have to recover the wallet and later on the client refuses to pay the money as agreed, they are at a big lost too. So yeah, and upfront payment doesn't look like a scam hunt, I think it is okay for them to request for it but it shouldn't be more than 30% of the total payment.