Hi
there are numerous payment methods today - more than ever. Also, there are various tools and software that measures the amount of liquids that have flowed through a certain point.
So there are available solutions, but in order to give you a proper, educated answer I would need to know how you currently deliver the fuel (fuel truck? Barrels?), do you do it yourself or one of your workers? How (online? Bank transfer? App?) and when do you currently collect payment? Also, what is the problem that you are experiencing.
What country do you work in? All the above influence which solution fits best.

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good luck