I've just invoiced a buyer for 2 magazines
Invoice No 89880
As I always give the option for both 1st & 2nd class postage, I use a % discount to get it as close to right as is possible, as a fixed amount doesn't work.
Although the invoice total is correct, & the invoice percentage is shown correctly as 29%, next to that shows the discount in money & that shows the wrong amount & doesn't alter when postage options are.