Wow that is the highest price I have ever heard of for a fuel system clean. Are they replacing your injectors too?
Normally what the procedure involves is the mechanic disassembling the fuel system to hook up to a "high-flow" or "high-pressure" cleaning machine that will use a fuel injector / fuel system cleaner liquid to thoroughly clean the entire system or at least the portion of the system under the hood.
Now, the fuel injector / fuel system cleaner liquid they use is not the cheap stuff you buy at the auto parts store. Most of those products sold there are inferior to what the manufacturers specifications require for a fuel system cleaner. A lot of shops use Protec in their high-flow machines:
https://www.protecautocare.com/motor-ve ... or-cleaner
Now, this is important. Some warranty terms and conditions require that all required/scheduled maintenance procedures happen within a certain period otherwise, they will void your warranty when it comes time to use it. I don't know if that is applicable to your situation, but you might want to check if you have a warranty on your Maxima.