Subscription products are gaining popularity as Shopify has recently released the Subscriptions API's and the number of Shopify applications that offer subscription features are increasing, such as Bold Subscriptions or ReCharge Subscriptions.
So are subscriptions apps compatible with Candy Cart?
Unfortunately not. Candy Cart is using draft orders to add free gifts to orders reliably. Draft orders are a native Shopify solution and despite that subscriptions are not supported for draft orders at the moment.
This has already been reported by other app developers in Shopify Community who are hoping that Shopify will integrate Subscription API to draft orders as well.
If you still want to be using the upsell pop-up on your store we suggest trying an alternative app from our portfolio called Candy Rack. It doesn't use draft orders so it is compatible with subscriptions. Also, the difference is that Candy Rack pops up when the Add to Cart button is hit while Candy Cart is triggered by click on Checkout button.
In case of further questions feel free to contact us at email@example.com, and we will be happy to help :)