Hopefully this helps someone ... the 6.4.1 OPC doesn'tt work with shipping taxes and the latest Sagepay payment module. Cre blamed the payment module. Sure. Anyway, here's a line of missing code in checkout_processing.php that fixes things.
So if you would like all of the order total modules to be passed through to your payment gateway from the OPC (as well as shipping, coupon, taxes etc lines), here's what works:
open catalog/checkout_processing.php
find line 99:
Code:
$order_total_modules->pre_confirmation_check();
and add below it:
Code:
$order_total_modules->process(); // added to make all order total modules get transferred through the OPC
(I posted this as a Fix to Cre, got an acknowledgement but they never posted it. So here it is again.)
Simon