Is buyer allowed to amend invoice amount on checkout? How is it done?

I am curious to know whether a buyer can add an amount to an invoice they've been sent. I know it is the seller's job to send a correct invoice, but in the case that the invoice is incorrect, is there any way a buyer can add to it at checkout (aside from using "send money" via paypal)?

---

Orignal From: Is buyer allowed to amend invoice amount on checkout? How is it done?