Good Morning,
I write AlabazWeb company. We have over 8 years experience in PrestaShop and we especilistas in developing modules for PrestaShop.
The module you need we already have for sale. It is called MegaCanon.
With this module you can add extras to your products general or independently for each options. You can decide whether these options are only informative and therefore mandatory, or optional. You can also indicate whether they are free (included in the price of the product) or if it has an additional cost. In case you want to apply an added cost, you can select whether to increase a specific amount or prefer to add a percentage to be applied depending on the product price.
Also, you can include a tax on the fee that has to be the same as the product. If the client decides to add more than one canon on your order, the price increases automatically on your bill detailing the unit price the cost of the fee and the final cost of the entire order.
We consider that fits exactly what you need. Besides, it is developed so that it could dispose of it today.
If you have any questions please contact us. Also, if you have to make any changes to the module, we can take care of it.
We await your response.
A greeting.