If you provide different types of services for your clients you might want to split them up into different Budgets. This will allow you to organize work in phases, per type of service, and more.
However, when it comes to invoicing, you will most likely want to invoice everything at once and be done with it.
In Productive, it is easy to create a unique invoice that combines different Projects and Budgets, as long as they belong to the same client.
Go to Billing > Invoices and click on "+ Invoice" to create a new invoice.
Enter the name of the Client you're creating the Invoice for and a new window will pop up, showing all Budgets on that Project that you can invoice:
Select all budgets that apply (note that you can only invoice Budgets in the same currency).
You can also add the name of the project to the line item, just select the Prefix line items with the Project name, the same applies if you want to add the name of the Budget.
Click on "Create Invoice" to generate the Invoice. This will bring you to a familiar screen where you can see the details of the invoice. Scroll down and you'll find a section that shows you which Budgets are linked to the Invoice and the amounts that belong to each budget.