I'm trying to set kilometer fee charges in my hr.expense module :

I created a product "kilometer fee charges" with five variants depending on the "tax power" of the vehicle. Each variant has a unit price.

When I create a new hr.expense, I fill the description field, then the product field. Here the product is : "kilometer fee charge (5 CV)". How can the product price automaticaly be displayed in the field unit_amount, under the product field ?

Which field do I need to edit or create to link the product price to the hr.expense template ?

Thanks a lot.