Store refused to exchange baby formula( Lactogen)

I ordered the wrong size baby formula online on the pick n pay app and when I phoned the customer care they said they dont do refunds online and i would have to take it to the store and they will exchange it there. The baby formula had not been opened and i just wanted to exchange it for the right size. The two stores i went to, to exchange the milk refused to exchange it and said i should exchange it online. Now im stuck with formula that i can't give to my child.