There is a toggle in the settings that takes one to a launch screen and then give 3 fuel choices as options for 3 different prices.

I already paid for removal of ads in the app. I do not see any other paywalled features either. So, what are those payment options for? Are they there just to support the dev? Also, if one donates via there, would it then remove the option of launching, thus streamlining the interface to only the basics?