Just install FastSpring Checkout Gateway for an inline checkout experience. This gateway handles everything behind the scenes to maximize payment performance and operate with full integrity. Features:

• Payment Gateway
• Include VAT/GST
• Sandbox Mode
• One-time Payments
• Recurring Payments (Subscriptions)
• Auto Subscription Cancel on Termination of Product
• Webhooks Support
• Subscription Update on Upgrade/Downgrade Options
• Refund Module Function
• Cancel Subscription (Admin Area & Client Area [Service Detail Page] )
• Show Subscription ID and Fetch Details (Admin [Product/Service Page])
• Optimized for the Latest WHMCS
• Install & Setup*
• Lifetime Updates
• Lifetime Premium Support
• Hook Available to avoid your clients being redirected to the invoice page after the order*

FastSpring Website: https://fastspring.com

FastSpring API: https://developer.fastspring.com/reference/getting-started-with-your-api

FastSpring Developer Login: https://login.fastspring.com/identity/auth/login

Contact Support to give it a try before you buy.

* = For Additional Charge