Sizes not the same as per the package

I went to Edgars to purchase a pack of 5 briefs for my 5 year old. I took the Mickey mouse pack aged 6-7 years as I know the sizes are quite small (generally). Get home,open the pack of 5 briefs only to discover that one of the briefs are not the same size as the other 4 and will definitely not fit my child. How is this even possible? I have paid for 5 briefs but can only use 4. Funny enough you can't return underwear. This is really dissapointing. Surely your staff check these things before they are packaged.