An add-on fee charges the customer extra when they use a specific option, for example a few dollars more for adding a name on the sleeve or a gift note. Customix adds the fee to the cart as a Shopify product variant, so the price, tax and reports work like any other product.
Step 1: Create the fee in Shopify
In your Shopify admin, create a product to hold your fees (for example "Personalization fee"), with one variant per fee and its price. You can hide it from your store's navigation, but keep it available on the Online Store sales channel so it can be added to the cart.
Step 2: Link the fee to an option
In the option set editor, select the option, scroll down and open Add-on fee (on some option types it's called Add-on price). Click the icon next to Variant.
Step 3: Choose the fee variant
In Select product, search for your fee product, tick the variant with the right price, and click Select. Then click Save.
When a customer uses that option, the fee variant is added to their cart along with the personalized product.