I don't have a problem with up-front payment:
I converted my paper based on certificate last year in the UK: I had carried out the necessary preliminaries with sending money to the CAA and getting my letter of authentication from the FAA.
I arranged to meet a very pleasant DPE from Florida at an airfield in Bucks who had prewarned me of the cost.
I turned up with a fistful of dollars (literally); she went through the paperwork, made sure it was correct and gave me a temp airmens' certificate and I handed over the greenbacks...........
The plastic Orville cert turned up 4 months later with my English endorsement.
I didn't have to go to the expense if going to the USA (and I had no plans for a USA visit in the forseeable future) and it was significantly cheaper than the cost quoted by the party referred to above.............
Sadly the FAA stepped in and clobbered any prospect of future visits by this pleasant DPE, for reasons best known to itself.
I think though there is still a UK DPE who can do this: details on FAA website last time I looked.
Cusco