You can generate promo codes on iTunes Connect. They're fairly limited: you can only get 100 promo codes per app version, and you're not allowed to use them for commercial purposes.
As long as you're not selling these cards, I don't think Apple would have a problem with them.
I have no idea if this is within the rules, but another option could be to use the "Gift This App" function to generate paid, non-expiring promo codes instead (send it to yourself). You'd incur the 30% App Store revenue cut this way in addition to any sales tax.
That's an interesting idea. I wonder what Apple would think. On one hand, they still get their cut. On the other hand, they like having full control over the experience. On the gripping hand, how could they even stop it?
Because if there's one thing we've learned, it's that developers doing things they're not supposed to do on the App Store always do a great job of keeping their "black hat" account separate and distinct from their "real" account.
Since apps are ranked in the App Store by revenue and number of apps sold, buying copies yourself could also be seen as ranking fraud, so I'd assume Apple wouldn't like it.
If you have an extra one and don't mind sharing, I've been waiting to get into stardew valley... If you could email it to me (in my profile page) I'd be so happy :)
As long as you're not selling these cards, I don't think Apple would have a problem with them.