Two different sizes in one box of shoes

I bought shoes online for my church service and they were delivered two weeks back, wore the shoes for the first time on Sunday and the discomfort I felt on my left foot was tremendous ended up taking off my shoes and I was barefoot the whole service only to find the left shoe is size 5 while I ordered size 6 so what they packed was size 5 and size 6. Im hurt cause now the shoes are already worn and they will not want to exchange the wrong size