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.
This work is majorly on trust looking at how hard it could be to give unknown person that claimed to have a wallet retrieval service without any proof. Once a service that is into wallet recovery has enough proof to backup is work, getting clients will be very easy.
Having this kind of work needs reputable because some wallet might contain huge amounts of money and the owner might not want to give it out to someone untrusted.