The PokéMileage Shop updates its stock more than it changes, varying on the amount of medals you have attained.
The items you can get when you have less than 20 medals are as follows:
The items you can get once you have more than 20 medals are as follows:
The items, however, do become considerable cheaper once you get more medals.
Other than that, no stock changes have ever been announced and I highly doubt there will be seeing as you get most Potions, drinks and generally Pokemon replenishing items from the store once you have acquired more than 20 medals.