Event organizers can add an additional item to an existing registrant's purchases in the admin user interface by referring to steps below: 

1. From Organization Dashboard (HOME), under Your events, click the desired event name
2. Click Participants
3. Click participant's name
4. From Actions list on right, click Purchase items ​
User-added image
5. Enter quantity or select options (depending on additional purchase item types), click Next
User-added image
6. Review modified items
7. Enter payment information
8. Click Next to complete process

Note: If add an additional item with waiver to an existing registration, your staff can collect signed paper waivers for add-on items from the customer and then note down the STATUS and DATE SIGNED in the Waiver agreements section:
AUI

If any registrants still have Unsigned add-on item waivers, you can send an email to remind them to sign any unsigned waivers by clicking event Dashboard > Quick links > Send waiver reminder link.