Fix incorrect payment amount requested when prices displayed with tax

On a UK shop (Zencart 1.5.1) where it is customary to display prices
including tax, Zen Cart adds tax to the sub-total. Since PayPal expects
the sub-total to exclude tax, this means that tax is added twice - once
by Zen Cart and again by PayPal.
Merge pull request #3 from billj9000/master

Fix incorrect payment amount requested when prices displayed with tax