We are flying into St Pete and would like to rent a car. We are driving home to Maine. Is this possible to pick up at airport but, drop off in Maine on the a specific date?

I would imagine since Alamo is all over the country. When making the reservation choose a different drop off location and put the closest one in Maine.

I picked up there and dropped off at a different Florida airport, so probably. You just can't pay with a debit card if you don't have a flight itinerary.