Guests can add a meal plan to their cart. They can compose their own formula. Type of sandwich, drink and dessert.
For example the list of drinks is a list of 'taxonomy' terms. However some drinks can give a supplement. For example the formula costs 1 € more if I take a coke.
I do not know how to do this possibility. I had imagined adding a field in the taxonomy of drinks where we can put the blow of the supplement. But I did not find with Rules how to add this extra to the cart.
Have an idea of how I could do it? Thank you for your help