I have only invoicing & sales modules installed. (no need for inventory)

   my company sells services like "car rental" , which has a different cost everytime i sell it (according to contractors i deal with)

i defined "car rental" as a service,the only option i have it to enter a standard cost to it (which is meaningless) , and i can't see a way to put a variable cost to it , or to change that cost for a specific invoice.

is there a way to go around that?

Version : odoo community 11