I'm sorry to hear that this is something you're experiencing! I see that you've already connected with one of our Support agents regarding your contract and the invoice in question. Given that this is an agreement between your client and your services, it's best to connect with your client in order to get this resolved.
Other remote workers here may have additional advice to share with you on how to navigate these kinds of conversations, but I'd also recommend checking out this blog post, which includes example conversations and templates for when to reach out about invoices and due dates.
I hope this gives you some ideas to start with.