I have a delivery customization app set up and when customers are buying subscription products, no shipping options are showing up in Shopify Checkout. Why is that happening?

If you are using a delivery customization app, like Pickeasy, then it could be that said app removes the shipping rate from Shopify Checkout. When buying subscription products, Shopify will only show the cheapest shipping option in the checkout. If your delivery customization app also filters the shipping options, then it probably removes the only shipping option Shopify provided.

You can check if that is the case simply by disabling the delivery customization in Shopify Admin > Settings > Shipping > Delivery customizations (see screenshot). After disabling it, a shipping rate should show up in the Shopify Checkout.