Dear PM4698,
Thanks for the offer, it would indeed be very helpful to have the onepage checkout plugin you are using (there are lots of different OPC plugins available!).
So to summarize, with the following steps you can reproduce the issue at your site:
1) Install the OPC plugin and enable it
2) As a new customer, try to order something. During the checkout, all customers are required to create an account.
3) For that initial order, use a ZIP code of, say, 1234
4) As the user just created, try to make a second order.
5) For that second order, you try to change the ZIP code of the address to 9876. (NOT a separate shipping address, but really a modified billing address).
6) The checkout page will correctly show the changed ZIP 9876, but the shipping plugin still uses the old 1234 address?
Are these the steps to reproduce the issue?
Thanks,
Reinhold