PayPal IPN Payments being received but product purchased not showing

Thank you so much for your assistance, Dr. Byte!

My hosting company send me a reply as below:

The cURL modules on the server (the ones that are allowed) can be used without any additional configurations. The only thing you should look at is the Outgoing Connections manager section, but I would suggest you to disable it entirely in order to figure out everything with the script.

Does this mean that I should add the IP 66.211.168.126 to my "Allowed remote IP addresses" list?

I'm not a techie person, and is having a hard time figuring out where is problem is. Your help is much appreciated!!

I'm not familiar with the particular control panel they're using. They're the experts on that, and should be helping you with specifics of how to use it and what to put in there and why.

As far as PayPal goes, I can tell you that the IP addresses used for Express Checkout and Website Payments Pro include the ones at this URL, under the "api-3t.paypal.com" and "notify.paypal.com" sections: https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/howto_api_golivechecklist

Under my store at Zen Cart control panel, should I set cURL proxy status to "true" instead of false?

No. It's very rare that a hosting company uses a proxy for CURL (and IMO nobody should). You would only enable that IF your hosting company specifically told you you needed it.