The best place to start if you need help with a specific product is to contact the developer. All WHMCS Marketplace developers have both a website and support URL listed.
Support Subscriptions Fixed typos Notification by SMS for customers about status of payments
Amplica.md already using it and love it Supporting credit cards, paypal, bitcoins Payout to paypal, bank wire and cryptocurrencies
CleanHTML Invoice and Quote Free template.
As WHMCS does provide tax rules options where you can configure your GST tax but we have some extra features in our module that makes it stand out and necessary to use.
Introduce extra billing models and charge your clients for actual server resource usage.
Send account statements to customers
Allow your clients to automatically login from Invoice Emails