Use discounted shipping cost instead of ship_cost.

PayPal will reject orders which do not have their totals line up.  If a
Free Shipping promotion is applied to an order, the initial shipping
cost will be used and the free shipping promotion will not be accounted
for.

This will cause the order to get rejected by PayPal because the order
total doesn't match the sum of the line items.

Discounted ship cost should be the correct value to use here.
Include line item adjustments in calculations

Line item adjustments were not included in the calculations, which would
lead to PayPal reporting an error that the totals do not match.

All adjustments are now included, and the item sum calculation is based
upon the the order total which is not shipping, or tax.
Add some comments to explain changes.