Tell them you will come back to pay the $20 but they have to cover the gas for your car and your standard hourly rate of pay for your time including travel time.
Do they think you're loaded with money? Because he does have a point, gas is not cheap. So not only are you considering giving them 20$ but you're losing out because you're wasting gas. For you to go back and return all of it is ridiculous and start all over is too much man. If I were you, avoid shopping from that place and just keep your gear and do not pay them the extra 20$.
Just because someone wasn't doing their job right, doesn't mean you should be the one losing out on it.
2012 Rome Postermania
2011 Flow NXT-AT's
2013 Flow Rival's