There are other ways to do this, but not necessarily simpler: - explicitly version every change to the store catalogue and require the version to be passed wit

There are other ways to do this, but not necessarily simpler: - explicitly version every change to the store catalogue and require the version to be passed with a purchase, disallowing purchases from old catalogues - make an expiring signed piece of data (like JWT) for each offer in the store catalogue and require it to be passed back with a purchase, accepting old prices briefly - make the purchase API two-phase, locking in the price on the first phase and letting the client notice that it changed and potentially neglect to go through with the second phase