Set the button to run a script:
Go To Related Record ( The Table Occurance That The Portal Is Based On ; A Table that the shows orders ; [Match Current Record Only] )
Note that you refer to seeing the 'order', but the portal you are clicking in is entitled 'Invoices' If you want to click on an invoice record but see an order record, that will be a bit different.
If you have a script that uses Go To Releated Records and it's only going to the first record in the portal, there's a good chance that something in your script is losing the focus on that record. Commit Records, for example will return the focus to your customer record and produce such a result. Feel free to upload a screen shot of your script if you can't figure this one out.