Add-Ons allow you to expand on selections by collecting additional information like time slots or tickets. They can also be used for merch since you can assign a price, set the quantity available, and add forms specific to add-ons.
Tip
If you need to collect money without having people register, Stripe Payment Links is a good solution for these non-tax-deductible payments!
Check out this clip from Planning Center University.
From the Manage tab, go to Add-Ons, and then choose New Add-On.

Fill out the information for the add-on in the way you want it to be shown to people during the registration process.

-
Enter the name of the add-on as you want it displayed to people when they're registering.
-
Add the price per add-on.
-
If there's a limited quantity, add that number. As add-ons are included in registrations, people will be able to see how many are left. It will show as "sold out" when the limit is reached.
-
If you don't want attendees to include add-ons when they register, you can choose to hide it on Church Center.
-
Allow attendees to make partial payments for the add-on.
-
Allow attendees to purchase more than one add-on.
-
Make the add-on available to certain selections, to tailor the registration process for each attendee and help prevent attendees from ordering something they don't need.
Select Save to include this add-on on your event.
Choose how your add-ons are displayed on Church Center.

-
Click on an add-on to edit the variations and settings
-
Click and drag add-ons to rearrange the order they're displayed in Church Center
When a person registers, they can choose add-ons available to their selection. The cost will be added to their registration total.

Update Add-Ons
After a person registers, they can ask an Administrator to change their selected add-on. An Administrator can change it in the attendee's registration details and send an email to inform them of the change.