I know WHMCS recently has a Mailchimp integration with abandoned cart, but personally I'm not really a Mailchimp enthusiast. There are better Email marketing providers like Klaviyo or MailerLite, who are also more focussed on the e-commerce automation side. The latter can connect with a third party module via WHMCS services, but unfortunately the automation part (eg abadonded cart) is not included. basically it only updates new subscribers now, but abadoned cart or retargeting etc isn't applicable.
Do others also feel that WHMCS can improve this? Look at Shopify and WooCommerce connections with many Email Marketing providers and create an all-integrated connection.