Custom field price and tax rules

[TaDrik]

Hello to all.

I am supporting a B2B site in virtuemart 3.8.9
I am using 4 rules in Taxes & Calculation Rules.
1 for the VAT (Type of Arithmetic Operation: TaxBill) and 3 for different shopper group discounts (Type of Arithmetic Operation: DBTax)
Some products (such as batteries) in my country, have a "Recycle fee" that must be applied extra to the product price. Unfortunately this extra fee is not always the same and must be calculated from the admin for every product. So I made a Custom field (String type) where the admin fills the price for the product.
The problem is that the price of the custom field changes every time a Calculation rule is applied. It must remain the same for all the Shopper groups.
Is there a way (maybe a hack) that can make the custom fields not to be changed by the Calculation rules?

Thank you in advance