Complaint

I recently bought my son soccer boots at mall@carnival and he wore them to one of his matches only to later realize that they were odds one size 6and one size 9 even though the box says size 6, I called the call center to try and get a remedy and spoke to a very rude Yvonne who told me since he wore the boots already nothing can be done but, my question is whose responsibility was it to check that correct sizes are packed in the boxes ? How was I to know the sizes are different