Zipify
Sezzle works with the Zipify app on Shopify, but you have to use the Shopify checkout and not the Zipify checkout. The upsell pop ups will work.
Subscriptions
Subscription payments don't work with Sezzle when you install an app that sells subscriptions on your products. Please contact Sezzle in case you don't want to show the Sezzle widget on these products.