Thank you, Ryan, for the information. It was helpful. I think I understand the problem much better now. I hope to have some time over the long weekend to try a few hacks to see what might work for PayPal. My intention will be to play around with other payment methods, too, to see if I can come up with a blanket approach.

Idea (?): After an order is finished, the cart is emptied at some point. I mean, if the customer returns to the site and checks his cart after completing an order, the cart is empty. I have not had time to look into the code for this yet, but perhaps whatever trigger empties the cart could decrement inventory first. Just thinking out loud...we'll see what I can figure out.