If you use Odoo v8 you can create an invoice from multiple Delivery Orders in the following way:
- Open the Delivery Order’s list view and select the Delivery Orders from which you want to create an invoice. The important think is all selected Delivery Orders to have the same partner
- From the “More” menu choose the option “Create Draft Invoices”
- Select the option “Group By partner” on the wizard that will show up
- Click on the button “Create” after filling rest of the fields on the wizard and you are done.
Please note that the Delivery Orders need to be created from Sales Orders in order to be invoiceable.