I would call on the phone and speak with a supervisor. It's really easy to blow someone off via email, not so much "live"
I don't think it's right that they are trying to get you to spend YOUR time and YOUR money to send the items back. s**t, you could've just kept them and never said a word. You should be rewarded for your honesty. The very least they could do would be to send you a box and prepaid shipping label (if it's UPS, they'll come and get it from your house, I dunno about any other companies). It shouldn't cost you a dime, and it shouldn't cost you any more than 5 minutes to put the stuff in the box, tape it shut, and put the prepaid label on it.
What's the status of YOUR stuff, btw?
If they cannot do more for you, that would be a shame cuz I'm sure you wouldn't want to order from them after that
Good luck!