Sales invoices can be produced using accounting software, they can be can be created using a word processor or even handwritten. Although they are often “easy” to amend, invoices should NOT be amended after being sent to a customer. Doing so leads to all sorts of issues, potentially for both parties. Payment may have been made for the original amount and a balance may be due, or the invoice may have been overpaid if the amendment was a reduction in the total. Depending on the recipient’s systems, the invoice might be both processed and paid twice in two different amounts!
Although it may feel a little unnecessary or a seem like “a bit of a pain”, please issue a supplementary invoice or credit note for any adjustments necessary!