The single most factor behind WP-WHMCS integration efforts is giving users a better ordering/buying experience. It is how visitors convert into 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) has beautifully designed order pages, which are feature-rich and user-friendly. WCOP offers ten different WHMCS Order Page templates/workflows to offer WHMCS checkout within WordPress, including its famous one-page checkout.