I'm wondering if there is any way, without building a fully custom app, that I can have a rewards system like smile.io, that also works if someone buys our products at a retailer?
We wouldn't be able to connect to the POS, so it would need to be a receipt scan or something.
This seems like a stretch.
I hope you are doing well! You can try AiTrillion. It's best for the Loyalty and Reward program. It's an all in one Shopify app and is fully integrated. Its loyalty program is integrated with reviews, email, web push notifications. With the help of the Aitrillion Loyalty Reward program, you can give loyalty points to your customers on different activities like signing up, visiting a store, making a purchase, allowing for push notifications.
And yes, it works well with Shopify POS. You can check it here: AiTrillion with Shopify POS
If you have questions, feel free to ask. I will be happy to help.