You are basically doing the right thing. Your "rights" are contained in all the purchase agreement documents you signed, plus the company's return/refund policy. This is basically a contract type issue, so general contract law will also apply.
I would suggest that you send a letter (keep a copy!) to the computer store. Recite the original agreement regarding the return. Also recite the date you sent it to them and include a copy of the UPS proof of delivery. Then tell them that they were originally to send back your money and completely cancel the sale. They have now requested 30 more days to do this. Then end it with a demand that said sale cancellation and refund of your money must happen no later than such-and-such a date.
Send a copy of that letter to the bank. You are sending a copy of this letter to the bank to protect your credit. OK?
Mark B. Replogle