As per your concern, our marketing platform integrates with Shopify POS.
Moreover, you will be able to set it up, so customers who registered on your store, are entitled to the mentioned discount.
Or the second option is to reward customers not the time they registered, but the time they subscribe. I mean when you provide them with a discount and use Shopify POS, you will be able to select the needed customer and reward him/her with the discount right from the Shopify POS checkout page.
The usual way to use the code is also available through your website.
Hope it helps.
You can set up this reward with Flits (https://www.youtube.com/watch?v=WkZ5e94QnWk). The app lets you reward shoppers through your Shopify POS and even send an automated email or web push to the shopper about their reward.