If you purchase the Early Math Adventure Bundle (2 apps included) or the Ultimate Expansion Bundle on PlayOsmo.com, the games will be unlocked forever on your myOsmo account. This means that the games can be downloaded and played on any device signed in to your myOsmo account, regardless of whether it is an Amazon Fire device or an iPad device.
If you purchase within the app, however, the purchase can only be restored and accessed on devices signed in to the AppleID that was used to make the purchase. In other words, any iPad that wants to take advantage of your purchase will have to be logged into the same AppleID in order to access the game. Purchases will also not be transferable to Amazon Fire devices.
Here's more information about purchasing the Plus games: Lettertopia and Math Buzz, or Counting Town and Shape Builder.