I would argue for a full refund. When i returned my TV they told me the same thing. I was ready to pay for it as the box was disposed of at the time (and they infomred me they would have to sell it unboxed). Seems a little harsh, considering I was told something totally different. Of course I assume you have the box etc?
I'm not sure if Richersounds is a fanchise, and if they are, then they will all have the same returns policy for in store purchases, meaning they should be consistant.
If they refuse to give you the full refund ask to see the returns policy, if it talks about the 15% handling charge then you are pretty much snookered. of course you could argue that as a consumer you should be made aware of the policy upon purchase of your item and I know for a fact the receipt does not mention this, as I have one right here. It can be tricky as they are not obliged to do anything as something people believe.
Still, be polite and I am sure they will give you the refund entirely.