Difference in behaviour between immediate successful paypal payment and deferred

Hi team, I have an unusual circumstance that we’ve unpicked over the last few months:

This question concerns a woocommerce site that offers Stripe or PayPal payments with an additional plugin: WebToffee’s “WooCommerce PDF Invoices, Packing Slips and Credit Notes”:

When an order is placed and immediate payment is successful via Stripe, the 3 x woocommerce emails are sent:

1. New order email to the shop owner
2. Order processing email to the customer (which the PDF invoice plugin attaches a PDF invoice to)
3. Packing list email to the warehouse (which the plugin attaches a PDF packing list to).

When an order is placed and immediate payment is successful via PayPal, only two emails are sent:

1. New order email to the shop owner
2. Order processing email to the customer (**though the PDF attachment is missing**)
3. **no packing list email is sent**

If an PayPal order is placed but cancelled (due to pressing the “cancel and return to merchant” button on the paypal login screen), no emails are sent. Fair enough – the order has not been paid for so no-one needs to know. It appears in the woocommerce orders list as a cancelled order.

However, if this cancelled paypal order is edited in woocommerce and the status changed to Processing, immediately upon saving the full set of 3 emails are sent:

1. New order email to the shop owner
2. Order processing email to the customer (which the PDF invoice plugin attaches a PDF invoice to)
3. Packing list email to the warehouse (which the plugin attaches a PDF packing list to).

I appreciate the WebToffee PDF invoice functionality is a third-party plugin and therefore it’s not clear who to ask about this issue, so I am initially trying to ascertain why there is a difference in email-sending behaviour between orders paid via Stripe or PayPal, and between immediate and deferred movement to the Processing status for PayPal orders.