Hey @Kmk 
First, I would recommend creating a CRM board that includes your customers and their contact information: monday.com as a CRM
From there, you can use the Quotes & Invoices app to manage billing for those customers.
If you specifically want to link this information with a different system like Rocket.chat, something you can try would be to use one of our 3rd party platform integrations to build your own integration with that system:
Otherwise, if you are comfortable coding or have access to any developers, you can build your own integration using our API:
If you need additional support on our APIs or apps platform, I would recommend doing a search for your question in our developer community here. You can also post to the community to get help from other monday.com developers.
If you need help building custom integrations, we also have an option for paid assistance through our network of certified monday.com partners. If this interests you, simply fill out this form and someone will be in touch shortly!
As for sending notifications to your customers, we have a few different integrations with different systems that will allow you to contact people external to your account. Here’s a few examples:
You can also check the marketplace for other systems we might have an app for already: The monday apps marketplace
You can also refer to this article for internal notifications: Alerts and Reminders with Automations
I hope this helps! Remember to mark as solved if this answers your question 
Cheers,
Jenna