This is Elias from Shopify Support. Thank you for reaching out.
I understand you're looking for a function to create a pop-up on the Shopify POS, so you can remind staff to include a promotional item when a customer purchases the qualifying product. While I'm afraid there isn't a function to add a pop-up or reminder through the Shopify POS. However, I think that's a great suggestion - I can see how a function like this would be beneficial for merchants and I'd be happy to pass this along to our developers. If Shopify moves forward with any changes or updates to the Shopify POS, you'll be able to find it on our changelog and updates page.
Though there isn't a way to achieve what you're looking for through the Shopify POS; however, since the inventory for the promotional items don't need tracking, there may be some options you can explore outside of Shopify. Perhaps, you can leave a mark on the package of the items or place a note by the cashier?
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