Exchange is how every delivery works
Every delivery we make is an exchange. Rodrigo arrives with a full 20lb cylinder, you hand him your empty one, and he takes it away. You pay the borough price, $40 to $50, at the door. That is the entire transaction.
If you have no empty to trade in, because this is your first tank or your old one went missing, add $40 and he brings you a brand new cylinder to keep. You pay that once. After that you are in the swap cycle like everyone else.
Why not just take it to a store yourself
You can. Exchange cages outside hardware stores exist, and if you own a car and live near one, they are fine. The catch is that in New York most people have neither. A full tank is about 37lb of awkward, valve-topped steel that you are not supposed to take on the subway, and that you cannot legally leave in a hot car for long either.
So the choice is usually not between us and a cheap exchange cage. It is between us and not grilling.
What condition your empty needs to be in
Ordinary wear is fine. Rust bloom, scuffs, a faded collar: all normal, all fine to swap. What matters is that the cylinder is intact and the valve is not damaged.
Cylinders do carry a manufacture date stamped on the collar and they need requalifying after twelve years, so an old tank may be at the end of its life. If yours is visibly damaged, deeply corroded, or the valve is bent or leaking, tell Rodrigo when you call rather than at the door, and bring a new cylinder into the plan instead.