To my knowledge you are correct, it’s not possible to buy PUTs in advance and use them later.
However there is this to consider: “The network on restart pays folk to re-publish any missing network data. (Network data is data that has been signed by a network section in the past and that is provable).” (source)
It’s not a big leap in logic to see that possibly we could maybe treat upload as separate to payment (ie all data is always considered missing). Whether separating these two aspects is a good idea or not… I’m not sure. It depends on what we consider the purpose of storecost to be.