It is important to note that ecommerce store owners should really use Express Checkout instead of Payments Standard, as it is by far the more reliable option. Payments Standard is much older, nearly completely outdated technology.
If you still plan on using Payments Standard, and want PayPal IPN to return to the customer to your store after paying, this Zen Cart Easy Help Tutorial will explain how you can do this.
Keep in mind that you must have a ‘Premier’ or ‘Business’ PayPal account if you want to use the PayPal module to take payments. If you already have the appropriate account type, you are clear to proceed. First, log into your PayPal account, and navigate to the ‘Profile’ tab. Once here, find and go to ‘Website Payments Preferences’. You should see an option for auto-return, which will likely be set to ‘off’. Change this setting to ‘on’, and copy-and-paste the correct return URL(The web address of the page you want customers returned to when they have finished their payment). It will probably look something like this: https://www.mysite.com/MY_SHOP/index.php?main_page=checkout_process. Please note that if your site does not have it’s own SSL certificate, the ‘s’ after ‘Http’ should be dropped.
There is another notable option on this page, the ‘PayPal Account Optional’. When this setting is enabled, customers without a PayPal account can still pay with their credit cards through the PayPal site without being required to create a PayPal account. Whether you turn this option on or not is entirely at your discretion.
Another important thing to note is that the sessions will work automatically. For this procedure, you do not need to change any of the sessions settings in your Zen Cart admin area.