I play for 2 weeks but I never bought fuel refill.
Why? Because it’s a waste. I mean you pay 30 tokens for 10-20 fuel (that is always left until the next level)
And if my fuel was 0 - I would still pay 30 tokens for a refill of 70 fuel. So you feel ripped off.
My proposal is - to offer users smaller packages of fuel like 10 fuel for 7 tokens, 20 fuel for 10 tokens, 30 fuel for 15 tokens, and so on.
This way - you get exactly what you pay for, and I am sure - the developers would get many more sales this way.