Bad service

I have always been a loyal customer to checkstar in overport Durban but I recently bought two packs of cuddlers nappies two weeks ago however i lost the slip... I sent my husband to exchange the nappies for either a bigger size or pay the difference on another brand as we previously did this with out a slip, the manager just scanned it and it obviously picked up the code and never requested the slip... Reason why we wanted to get a bigger size was because this size was just too tiny even though it said up to 17kgs.. Fast forward to today the 6th of may.. The store refused to assist us at all despite our constant pleas as i mentioned they had done this before with out an issue... Im now sitting with nappies i can't use and i will not be shopping there again.. I always ask the staff about these things prior to purchasing and have never had an issue until today.