This has been asked numerous times and without getting technical it has been done quite successfully with a 12v car battery and a pair of wires after...
You buy say 100 credits and if you use a 10 credit app it drops each time you use one. SO if you do something and it DOESNT work it is another 10 to...
There is a difference between a hub centric spacer and a wheel spacer though. One fills in around the stub to enlarge it to fit the wheel centrally....