Prestashop 1.7.

I'd like to add engrave services to product page, and modify the price depending on the number of characters. The characters will be inserted by the customer into a specific field. There will be specific prices, depending on the material (ex. woof, plastic, metal). In any case the number of characters * (specific price) will be added to the product price. I need the option to select in BO the products for which the service is available. Also from BO possibility to change the per character price.