Although there are many powerful features revolving around invoice lines that allow you the flexibility of manipulating discount amounts and percentages as well as unit prices, all at line level, sometimes you just want to give a fixed amount discount as the last line of the invoice.
This is how you do it:
Firstly add all the lines you want on the invoice, then add the last line as an adhoc item with a description of whatever you like but maybe 'Invoice Discount' would be appropriate.
Give this adhoc line a quantity of one and an amount of zero. Now click on the last line created in the invoice (the adhoc item you have just added) and use the discount button (F6), to set the discount amount for the line.
This line will now have a minus amount for the discount amount, and will cause the entire invoice total to reduce by the prescribed amount.