Hi Edward,
Ryan's right - there's something not right about what's happening here. This import should complete in a couple of minutes at worst. Please copy the data to a local drive and see how long the process takes. (Or better yet, copy the data to your machine to remove the variables of your client's compute environment entirely.)
Get back to us with the results.
PS - Invoices has never had a direct link to Inventory as it has most commonly been used in an environment where there would be little benefit in that tight an integration. Our minds are open to being changed however!
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Andrew Bates