Do you think anybody would pay for wallet recovery service before the fund in the wallet can be recovered? I don't think so and it's only someone that is not sincere will want to get paid upfront before a wallet can be recovered.
I have seen cases of people that looks like scammers asking for upfront before they could work on wallet recovery cases. I believe if fund is inside the wallet, after recovering the fund in the wallet, the percentage agreed rate can be withdrawn after the job is done.
You're very right, as I have said earlier wallet recovery is one of the delicate functions and I think it looks extraordinary if we should consider some stuffs as cryptocurrency (Bitcoin) enthusiasts so 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.