The customer has ordered catering for the next event but has forgotten to add upsell items to his catering sales.
He is now asking you to add these upsell items so that they are included in the catering for the next event.

The first thing you should know is that the upsell items can be found under the group "Shop".  
Here's how you can check the group and how you can add upsell items.

1. Click on the tab "Products" -> "Shop" -> "Shop"  

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

3.  To add upsell items 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 "Upsell" tab.

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 upsell items should be added and click on save.

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 € 259 (the value of the upsell taken)