30 day rule should have exceptions.

I bought shoes about a month ago for my daughter but it turned out i bought a smaller size, so after misplacing the slip i eventually found it and when i went to the store for the exchange i was told i can't because the 30 day period is over I found that very unfair as the shoe is still in the box and i have the slip as proof on top of that i don't want any money back i just want the correct size