No set aside

I bought my daughter frozen flip flops but unfortunately I ordered the wrong size. I want to exchange for a bigger size I can't and when I call the call center the agent tells me it's "2 different" orders as the shoes are grouped differently therefore different barcodes. my problem is that they are running short of stock on the bigger size and I am most probably not going to get the bigger size shoes. I have the smaller shoes in the packet, haven't been open and for good customer services I was expecting them to be able to set aside the bigger size so that when they get my return/exchange I am guaranteed the bigger pair. I am a very disappointed customer after getting such good service from them. they are not willing to set aside a pair of shoes for an exchange. another option would be that when the courier fetches the pair i have, they bring the bigger pair, inspect and then exchange immediately.