There are two different approaches to apply extra fee when a field is being used by your customers. One via Shopify variants directly, and the second via add-On product created in Shopify.
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You can create variants called "Yes for personalization", "No need for personalization" in Shopify Admin for selected products. You can edit the price for that "Yes for personalization" variant in Shopify Admin easily. Then in the app only apply the fields for "Yes for personalization" variant for those products. By doing so, our fields will be only displayed on those particular variant and those variants would have extra charge in place directly in the price title by Shopify.
With this method, customers cannot remove any extra charge at all in the cart or checkout.
You need to create a product in SHOPIFY ADMIN to represent your extra charge on cart and checkout page. Then search this newly created product in the selection section in the app. If you cannot find the item, please click refresh button to retrieve all your products again.
The newly created product must have sufficient stock. Ideally "Quantity Not Tracked" or "Continue Selling When Out of Stock" selected in Shopify Admin.
The newly created product must be in active status, available in online store sales channel too.
