So this is my setup (and can’t get my head around a solution):
User creates a product [cost 30] and defines field “fee ” or “no fee ”
Customer buys product and in the two scenarios is presented with:
A. product 30 + fee 10 = sum 40 (element is visible when user defined fee , when product was created)
B. product 30 + fee 0 = sum 30 (element is visible when user defined fee , when product was created)
Because the fee  needs to be included depending on the visibility of the fee element, I can’t just use the DB value (because then it’s always included visible or not visible). To solve this I created a temporary field fee [temp] which should copy the fee element value in the DB and be included in the sum:
when element fee  is visible; product 30 + fee [temp 10] = 40.
Finally, another workflow would change fee [temp 10] to 0.
I think this is a valid solution but can’t get fee [temp] to copy and save to the DB.