Fix down cast error

[Letalus]

Within the latest release we have in line 219 of the file packages/linkfive_purchases/lib/logic/linkfive_purchases_main.dart a down cast error --> reason: Down casting is unsafe and therefore is not allowed anymore:

https://stackoverflow.com/questions/72350214/flutter-type-a-is-not-a-sup-type-of-type-b-even-though-b-extends-a

This pull request should fix the issue. #8

Disclaimer: The handling of the sku needs to be reworked soon as well