Most invoices appear on their own: one with every sale, then one per billing period for a recurring contract. This guide is about the ones you handle yourself, and about the two ways to correct an invoice that is already out.
Linking a contract matters for more than the price. Only contract-linked invoices count towards the access rules in When a member stops paying; an unlinked one chases by email and nothing else.
Change amount rewrites an open or overdue invoice in place, and it always asks for a reason. That reason is the difference between an invoice history you can read a year later and a number that changed for reasons nobody remembers, so it is required rather than optional.
Payments already taken are the floor. Going under them would manufacture an overpayment, so refund a payment first if the amount really has to drop that far. Setting the amount to exactly what is paid marks the invoice paid, and setting it to zero writes it off.
Void an invoice that should never have existed: a duplicate, or a sale that fell through. Voiding is for invoices nobody has paid; once money has arrived, refund the payment instead. A void invoice stays visible, because deleting the record of a charge a member saw is how disputes become unanswerable.