That's easy to solve. Tacos and other Mexican food is a nearly endless source of farts, which contain a great supply of methane. The drone could deliver a taco and a small yet expandable fart collection device. (Don't laugh, some places use such devices to collect methane from cows who wear a collection tank on their back.)
The Taco delivery drone would be told to make a fueling stop at the customer's uhh... "rear entrance" where it would issue a credit for the fuel to be applied against future orders.
There would also be an incentive for the more flatulently gifted to collect their emissions and get highly discounted tacos.
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