how do i do a postage discount?

[freefruit]

I want to take a pound off the postage and am at the invoice stage but can't see anyway to do it I have tried rewriting the 3 pounds to 2 and taking off 1 pound and getting it recalculated but it keeps defaulting back to the 3 pounds
any advice as to what to do next?
thanks

don't recalculate.

just change the amount as you have been doing, preview if you want but don't recalculate

thanks