Add-ons are any product included in a survey collection that have not been defined as a ‘reward’ via product type. Projects with the highest add-on sales offer products that are relevant to their Kickstarter rewards (i.e. you probably won’t sell many t-shirts as add-ons if you’re project was a kitchen tool.) Offering the items from your campaign for purchase during survey completion is the simplest way to get add-on revenue.
Add-on items do not require any special identifiers to be offered within survey collections. Simply add the product to the collection and you're good to go.
If you would like to offer reward items for additional purchase as add-ons then you can add the tag 'Upsell' to applicable products to automatically include them in the add-on/upsell section of the survey.
Any discounted price for add-ons (usually shown as a sale price on most stores) will be displayed as a Backer Price on survey pages.