Sale Not Recorded

A sale failed to register in our newly upgraded (from 4.3.4) CS-Cart 4.3.5 store. It came to my attention when the customer told us she did not receive an email confirmation for her purchase. What is odd is that we did receive a payment in correct amount to our paypal account but there is absolutely no record in the cart for the customer nor the order. The product shows 0 in stock now, which is correct since we had one in stock before the sale. It is possible, but I can't confirm, that CS-Cart's in-stock value was incorrectly set to 0 before the sale. Would greatly appreciate any ideas as to what could be responsible.

The order is most likely in the Incomplete status. They don't show with normal orders, you must click the Drop Down Arrow w/Gear Icon on the Orders page and choose "Incomplete Orders" from the list.

PayPal orders end up there if you don't get a response from PayPal that the order was paid. You can solve that problem (in most cases) by completing the Return URL for at PayPal. Instructions for doing that are the last item on this page:

Although the instructions include a path to the form that I don't have in my PayPal account, once you find the form, the rest is correct.

Check also under Marketing tab and abandoned carts. It happened to us before