Exchange 2 CLOSED packs of nappies for bigger size

3 days ago I bought a box of Huggies pants no 3 (size that I thought will fit my baby) and they are to small. I asked if I can maybe changes the 2 remaining SEALED packs for a bigger size but they refuse because the box is open. They say I should have bought the correct size, but according to sizing on the box it should have fit my child. This is the first time buying pants. Can't they sell the perfectly sealed packs again?

I support Baby City every month and this is what I get.