The single most factor behind WP-WHMCS integration efforts is giving users a better ordering/buying experience. It is how visitors convert to customers.
If you use WordPress & WHMCS, then WHMCS Cart & Order Pages (WCOP), can be your best choice for your WHMCS Checkout for WordPress needs.
WHMCS Cart & Order Pages (WCOP) have beautifully designed order pages, which are feature-rich and user-friendly
WCOP offers 10 different WHMCS Order Page templates/workflows to offer WHMCS checkout within WordPress, including its famous one-page checkout.