Fix for incorrectly formatted item amount

When you need to apply negative fees or taxes to the order and the customer wants to pay with Paypal, Paypal will show an 'incorrectly formatted item amount' error and the payment will fail.

The problem is that Paypal doesn't allow to use negative (i.e. -10) values for amount_x fields.

With this new option, the store manager can force WooCommerce to send the order total as a single amount_x field (just like it does when prices are including taxes).