Cashier who does not know her job

I bought clothes online (eft) on 30 March which some were on promotion and on the 7th of April I received an email indicating that they have been put on hold due to one item being out of stock. On the 21st I went in store to enquire about my order and found that they have been delivered except for the unavailable item. Today I checked online for stock and found that the store has both items that I needed to exchange for a bigger size available. The cashier (Monsly) told me that 'one item is not available but is in the boxes at the back. Since I cannot use my invoice more than once for exchange/ refund, she is going to exchange the one that is available and refund the other for R118.68. If and when they decide to unpack the items in the boxes and by chance I pass by and check for the item and find it on the floor, its only then I can repurchase the item in the bigger size for the actual price of R259.99'. Meaning I will have to buy the same item, which I initially bought in full, for an extra cost of R141.31. I declined and told them that I'm not going to pay anything extra for items I have already paid for in full. Is either they go looking for the item today so I can exchange both today or they call me the day they decide to take out the item out of the boxes at the back. As I type I am waiting for the Manager's intervention.