Allow for purchase of all arrangements of a song at a discount
This isn't specific to the Windows app, but neither Song Requests nor other sections seemed like the right place for it: like many customers I was surprised and disappointed to find that my monthly five included "songs" in my membership weren't actually five songs but five arrangements. So to access all of one song's difficulty level, you have to redeem 3-6 "songs" credited to your account.
I think the normal expectation after taking lessons that gradually up the difficulty as you learn that the songs you buy would do the same. That is, you'd start with the rookie versions, progress to the intermediate versions, and then end with the hardest advanced version.
I think the ethical thing to do would be to give all arrangements of all purchased and redeemed songs to all members, but failing that, could there are least be some discount so that two or so "song" credits could actually get you the full song?
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Kevin B. Walker commented
If the goal of piano sessions is to progress from rookie level to advanced level, then why would anyone want to buy the rookie (or intermediate) version of a song and then have to again buy each succeeding level as their skill improves? I think that you should package all three levels of each song as a single, discounted purchase for members, or simply offer the most popular (advanced) published version for purchase, and practice until you can play that reasonably well.