My name is Lizzy and I’m on the team at Shopify.
While you're correct that the Buy X Get Y discount is not currently compatible with the Shopify POS, there is a way to still create the effect of the discount.
When completing an order you can tap on the product you'd like to discount. A pop up will appear that gives you the option to apply a discount. Here you can set either a percentage or dollar amount as a discount and apply a name. This recording shows the steps.
You will also be able to view the name for the discount on the order if needed for future reference.
I also wanted to thank you for sharing your feedback with us. I've made sure to let our development team know that store owners are looking for better Buy X Get Y discount integration with the POS app! We appreciate feedback from our merchants as it helps us to constantly work and improve our platform. Are there any other features of Shopify you had feedback on, that you'd be interested in sharing with us here?
Please feel free to let me know if you have any questions about what I've said!
I also just discovered this issue as I'm setting up for a convention. This creates a huge burden not having this discount type available to use through the POS. I expect to do over a hundred orders a day over three days, and most will use a (2 for $X) type of discount. Please get this fixed!
It would also be helpful in the mean time if there was some clear mention of this not working on the POS in the help files.