Basically, if you're already paying ~$14 for those 7 extra retainers, but you want just one more, you have to pay ~$5 for the companion app and then another ~$2 for the extra retainer.
So yes, it is that stupid.

I don't mind the app having a premium option. But that option should: INCLUDE the extra retainer, not just allow you the option to buy more. And also not give you an upper hand in playing the market with the kupo nut currency.