Generate a single Invoice for multiple Productions Orders

My goal is to create an Invoice for several Production Orders depending on each customer.

The Workflow that I want to adopt is the following: when a customer request a job, a production order would be generated. At the end of this Production Order, an invoice for that customer would be generated. The Invoice may refer to one Production Order or several Production Orders, with the amount of the entire work.
Additionally, a Budget could precede the Production Order, depending of whether the customer requires it or not.

I must also say that I use a personalized doctype for production order, not the stantard doctype.

How can I perform this process?
Thanks in advance!

You should not require any customization in this. Production Orders and Sales Invoices are both made from Sales Order, so you should be able to map your scenario if you consider Sales Order as you reference.