Edit fields total and taxable in billing

Good morning ,
I found this great open source and I installed it right away I find it well done and congratulations to sviluppatori.tuttavia I would have a problem to solve :
having a gas fuel I need to bill for many customers at the end of the month , so the price of the product varies from day to day, so I can not just physically enter the amounts ( total and taxable ) in the screen of invoices how can I fix this problem ?
thanks in advance greetings