Offered discount not given

Hi,

I recently placed an order from an online retailer who then added a bigger discount on their website (I bought at 30% off, they then increased the offer code to 40% off)

I requested a refund of the difference, however the response I received offered a much larger discount, and if I was happy with that I should respond (the culminative discount would have worked out 60% off, not 40%)

I replied with glee saying that's great, then was lost in the system for a week, I replied again, somebody picked up my email, said they'd process this immediately then sent me the original amount of an additional 10% off.

I did complain that what I was offered isn't what they have refunded, and they've stated this was a mistake, but the wording in the response was very specific, so I can only assume they've backtracked on their original offer that I accepted.

What should I do? I've had a few round robin emails with them explaining that they offered me more than I had asked for, but they are maintaining it was a mistake even though I had actually accepted their offer and not mine.

I purchased through PayPal, so I can open a case with them, but I don't know whether to just leave it, or press on and see if I can get what they offered.

Your opinions greatly appreciated!