Can customers buy add-on products directly?

No. Add-on products only exist to support paid options, and Optionize prevents them from being bought on their own.

Two protections are in place:

• Page access: if someone opens an add-on product's page directly, they're redirected to your home page. (You can allow viewing these pages with Allow direct access to add-on products in Settings, but they still can't be bought alone.)
• Checkout validation: a checkout rule blocks any cart that contains an add-on product without its main product. The customer is asked to remove it before paying.

So even if an add-on product URL leaks into a search engine or someone crafts a cart link, the add-on can't be purchased by itself.

For what these hidden products are and why they exist, see Hidden Optionize products in my Shopify admin.