Ability to change Invoice after sent

I would love to be able to change the invoice after I send it to the client online. As well as add my own custom fees. I had a customer recently last minute want to only do one session instead of two and the system wouldn't let me change the invoice because I had already sent it earlier. I also cant add my transportation fee which is different per customer easily at all. The tax also is applied to all items but where I live only portions of my work are taxable so I'd love to pick what is taxed and isn't.