Hi Dean,
Just another thought... in that situation, surely you also have inter-company billing from the supplying Company Code to the Sales Company? If so, could it better to produce the Export Declaration from the I/C point of view? As far as I know, that can be achieved by simple configuration - in the Billing Type definition, just set the SD Document Category to '5' and the Transaction Group to '8'.
You might also need to adjust the Pricing Procedure so that the Sales pricing is used for the GTS value.
Regards,
Dave