Why the upsell order is “Pending” when a customer uses PayPal to make purchase?

Do you know why the upsell order is pending when a customer uses PayPal? If

a credit card is used, it works fine. However, if PayPal is used, the

initial order goes through and completes, but the upsell order is pending.

Solution: I suppose it happens because you don’t have Reference Transactions enabled in your merchant PayPal account. Here is a little guide for you on this topic:

https://bogdanfix.com/how-do-i-get-reference-transaction-enabled-with-paypal/

Alternative question: Why is only the original / parent order / first payment coming through properly, while the upsell orders are not paid / are pending?