Can a microwave work without having a door on it?

Can a microwave work without having a door on it? Our door was warped from gas burner below. We ordered a new door with the understanding that it could be returned if it did not work when using microwave. I was then told after trying to return it for the money to go toward a new microwave. I was then told they could not refund our money since the door was not essential for microwave to function. Is this a reasonable reason for not being able to return a new microwave door that was never used? Please respond as I just do not understand their reasoning for not allowing us credit .

/ Thank you, Judy O'Rear