like everyone else, i sympathize. however, that's a purely emotional thing.
in fact the shop is not at fault here. nohow, noway, no ifs-ands-or-buts.
certainly, it's a highly desirable customer service courtesy to post return policy signs where customers can see them before handing over the money, but they don't have to.
the shop owner is not responsible for searching your accessory collection before selling you a foot.
the box was opened, so she'll have to resell it at a discount.
the merchant met you as close to halfway as possible.
you may not have gotten what you want, but you will not end up emptyhanded.
again ... not the shop owner's fault.
case closed.