Nowadays most of the services which include FRP, Pattern, Pin Unlock services are done Online either through Teamviewer, USB Redirector etc.
More or less, the service provider will ask money in advance.
1- If the Source is Reliable, you can pay in advance.
2- But if the source is new you should not pay in advance. You should pay after service to avoid any fraud.
3- You can make a claim if the source has been renown by most of the people. But, if the source is not identified by most of the people then you should avoid business with them.
Some of the cases with pics: