twice this week now, i have created a combined invoice, with just a single postage charge.

The invoice comes up with the right totals for the items and the single postage,  and the customer gets the right invoice,  but when they click on pay it now,  it comes up with just a random amount to pay based on just one or 2 of the items.  I have done heaps of combined invoices, so I know we are doing the right thing.  Any ideas