What about using Commerce Gateway for that? I know - this is expansive (Biztalk license, SAP Connector, development etc.) but it is "standard" way how to...
I'm sure this has been done many times before (I know of at least one customer that we helped connecting to SAP) in many different ways. There is no MS supplied standard connector between Navision and SAP though, so it's always custom development. Even with CG you are probably looking at some degree of development, if you'll be adding document types that don't come with standard CG.
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