Troubles with an online retailer

CYPER

The quick story:

1. I ordered a laptop bag from Play.com
2. They sent me the wrong bag
3. I called them and they promised to send the right bag but I have to return the wrong one. Got an Returns Authorisation Number.
4. I got the wrong bag again so now I have 2 wrong bags
5. I called again and they said the trouble comes from their warehouse and I should order the bag I want from somewhere else. They promised to refund. Got a second Returns Authorisation Number.
6. I ordered the bag I want from Amazon and everything went smoothly.

So basically now I have 2 bags that I have never ordered and £29.99 less in my credit card due to a Play.com error.

They insist I should send them the bags or they will not refund and even charge me for the second one as well. The problem is I come from work after the post office is closed.
What should I do?
They can't just demand to waste my time only because they made a stupid error twice. I paid for something I never got, but got something else I didn't pay for.

Please help. Thank you in advance.

jonesMUFCforever

Post it back at the weekend!