If a customer adds a post-purchase upsell to their existing order but the payment fails, Shopify will still add the product to the order even though the transaction didn’t go through.
Unfortunately, this leads to a “Partially Paid” order status due to a limitation in Shopify’s API.
As a result, apps like Moonbundle (and any other post-purchase apps) can’t fix this issue unless Shopify updates their API.
The entire flow from payment processing to adding the product is fully controlled by Shopify, leaving apps with no control over this behavior.