I do the same. Paying a little extra is worth the peace of mind of not having to worry about it as you're heading to the airport.
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We took ours back full, we filled up at a gas station just before the OBT/417 entrance. Guage still showing full at airport.
We did that in Febuary, completed details online in UK and as we had a reference number was told by Alamo rep standing by machine to go straight to garage. Took a bit longer to go through at exit but not problem.
We had a Silver Basic booking (1 driver) not sure how it would work for extra driver.