merge invoice

Does anyone knows how to code an API to merge invoices?

I found how to create api:

http://www.whmcsjet.com/introduction-to-custom-whmcs-api-functions/

http://www.whmcsjet.com/how-to-check-existing-client-in-whmcs-by-email/

And would love to create something like this:

https://requests.whmcs.com/responses/api-function-merge-invoices

I think it would be very useful to reduce the number of invoices

I understand that in some countries you are allowed to play with invoices but in many others you simply can't. Belive me or not but basically in every single EU country you can't change an invoice and this is just one thing. Also, why your clients should be forced to pay this kind of invoice? What if they don't want to renew a specific line? What about next due dates and invoice date?

That said, I agree with you but but in my opinion there are better ways to reduce the number of invoices. I personally use a method that allow me to create a maximum number of 12 invoices per year (1 big invoice per month per client) in respect of tax regulations of all countries.