When you set an option as a paid field, the additional cost is automatically added to the cart whenever a customer selects that option in your online store. If you use a percentage paid field, the amount is calculated as a percentage of the current product price at the time it is added to the cart. This lets you easily charge for add-ons or extra features, and the customer will see the additional cost included in their order at checkout.
Bundles
Paid fields don't work on products that are set up as bundles. Paid fields add the extra cost with the same Shopify feature that bundles use, so the two conflict and the paid field price is not added to the cart. Fields without a price still work on bundle products. If a product doesn't need to be a bundle, remove the bundle setup and the paid field price will be added to the cart.