This from a colleague:

>>
You can do it, almost...
We have a system that integrates with WS during the shipping process,
which sends the order# and shipto address, and returns the freight charges
and tracking # to the SO. It uses an ODBC link to an Access database as a
conduit. It does require the operator to go into WS and key in the SO#.
which our mod prompts them to do. But I think that the latest WS (10.0?)
will allow all that to happen automatically. If you dig far enough into the
UPS tech support team, you should be able to find someone knowledgeable
enough to point you in the right direction. They are very supportive of
these solutions because it helps them lock in their customers.
>>

I agree about the quality of their tech support. If you don't get the answer right away, keep digging.