The way you worded it sounds like you'll only pay $15. Unless they speciffically specified you pay BOTH ways.
I know when I had to RMA a HDD to Seagate I had to pay the return shipping. It had to be insured and through a trackable provider, ie UPS. So in the end I paid almost 10 bucks to get it shipped. I would assume with a graphics card they would want the same standards. If it something they can fix they can re-sell it as a refurb so they want it in a good condition.
$15 for return shipping but it will probably cost me $15-$20 to ship it to them. So actually this will probably cost me more like $30 - $35. My psu costed a little over $20 to ship. The box will probably be about the same size but lighter so I'm guessing a little cheaper to ship.
But its not uncommon to have to pay shipping both ways?