Hikashop not receiving payment update from Paypal - One specific account

-- url of the page with the problem -- : quoitsusa.com
-- HikaShop version -- : 4.0.1
-- Joomla version -- : 3.9.1
-- PHP version -- : 7.2.6
-- Browser(s) name and version -- : Safari 12.0.3
-- Error-message(debug-mod must be tuned on) -- : No error message

Hi, I'm currently having an issue that I'm sure is in the Paypal side, but I'm hoping you can give me some direction based on your experience. I have built a store in Hikashop and Joomla, and everything worked fine using Paypal sandbox for testing. When I moved to the production Paypal site, transactions worked properly when using my Paypal account, meaning the order was marked as "confirmed" after the payment was completed. However, when I move to the client's Paypal account, no update is received from Paypal, and the order remains in the created state. The payment does complete successfully in Paypal. In the debug log, I only see the payment go to Paypal, but no response is sent back. The client previously used this account for a different site, so I confirmed with them they didn't have the IPN set up, but it is currently blank. Any assistance you can give me on where in Paypal the replies would be blocked? Thanks in advance.

Thanks. There must be something weird with the client's PayPal account. According to PayPal, we were required to put our website in the IPN configuration, even though the transaction overrides it. I've also not needed to do it on other accounts, but figured I would post the solution for those that search in the future.