Unable to Add Products to Cart
Symptom
Clicking Order Now on the shopping cart product selection page (cart.php) or following a direct product order link (for example, https://example.com/whmcs/store/1) appears to refresh the page and shows the product selection page again.
When this happens, the ordering process may not proceed to the product configuration or view cart stage.
Cause
1. Accessing the shopping cart using a different URL from the one specified in the WHMCS System URL setting.
2. This issue may be due to an invalid URL (missing the slug) in the product group's configuration.
Solution 1
Small differences in the WHMCS System URL and the URL used to access the shopping cart can cause this behavior. To resolve this:
1. Note the URL displayed in your browser on the shopping cart page.
2. In the WHMCS Admin Area, go to Configuration > System Settings > General Settings.
3. Adjust the WHMCS System URL value to match the URL being used to access the system (for example, https://example.com/whmcs/)
4. Click Save Changes.
Small differences such as the protocol (https:// vs http://) and subdomain www. are important.
Solution 2
A missing product slug can use this behaviour. To resolve this:
1. In the WHMCS Admin Area, go to Configuration > System Settings > Products/Services.
2. Edit a product group.
3. Enter the desired product group URL (slug) in URL.
4. Click Save Changes
5. Repeat the process for every product group.