Failing to pass Postage to Paypal

Hi

We're using OpenCart 1.4.9.

We have had a couple of orders where the postage cost has not been passed to paypal, so the customer has been undercharged and we are loosing money.

For instance:

We have an order which should have been £12+£8 postage - OpenCart correctly processed these values and the customer invoice shows this - HOWEVER - only £12 was charged by Paypal, so the postage value wasn't passed to Paypal. We have therefore lost £16 on the total order!!!

This discrepency is a big worry, we've gone through the coding and cannot find a reason why this would have happened, plus its processing other payments fine.

Help and advice appreciated.

Thanks