US prices. The App Store and Google Play show your local price and add any tax. Credits are bought in the app.
No. You buy credits when you need them, and they never expire.
It costs nothing. If a video fails, or takes longer than an hour, its credits come back automatically.
5 credits. If a shot came back wrong, the fix is free.
Apple or Google handles refunds of money, under their rules. Credits can't be exchanged for money.