I would let the store know how much waste you had. What is the worst that can happen in letting them know, I am thinking they would do nothing. Best case scenario, they may actually give you a store credit. I have bought a fair amount of wide back and have never had it as bad as that. I think the most I lost to squaring up was 6".
I usually buy extra as well and usually get my backing on line. The fabric is pretty and you can use the waste bits in scrap quilt.