285 vs 275

I bought new tires today. nokian outpost nat. I wanted 285/55r20. when I called and ordered that's what they quoted me on, and what I thought I was getting. when I got home I looked closer. they installed 275/55r20.

should I care about 10mm? they offered to swap them, but want me to pay the difference. ($500 cdn) or I could just return them and get something else.

Im not sure why it's bothering me so much. i might not have gone with the 285 had I known the price difference, or I may have shopped around for some at4w?

what would you guys do? keep them, make them swap and try to get them to honor the price I was quoted? other options?