The customer took a catering formula during the last event and he has taken extra drinks during that event. 
You need to add the extra refreshments to the catering invoice so that the customer will only receive 1 invoice.

The first thing you should know is that the consumption items are in the group Catering Consumptions.  
Here's how you can check the group and how you can add consumptions.

1. Click on the tab "Products" -> "Catering" -> "Consumptions"  

2. Click on the green "Add" button if you want to add consumption products/items to the group.

3. To add consumptions to a catering sales you need to go the Catering tab and click on the green "Catering" button.
4. Search the customer, make sure you have the right event and go to the "Consumptions" tab.
!! This is only possible if the match is still active.

5. In the Quantity table, fill in the number after each product that needs to be billed.

6. Return to the Catering tab, click on "Add to open sales", choose the contract to which the catering should be added and click on save.
!! This is only possible if the invoice for the catering has not yet been made.

7. In the contract you now can see that the extra usage has been added to the tab "Items".

8. And in the tab "Instalments" you can see that the amount is increased by € 130 (the value of the consumptions taken)