Done a lot of work over the years integrating UPS WS to Mas90 - very first thing to decide is ...
Does the invoice get created before or after the tracking number...
If the invoice gets created BEFORE the tracking# then the best you can hope to do is import tracking numbers into AR history and to effectively do this, you will need to know the Mas invoice number that goes with each tracking number. I have had success entering the invoice number into a UPS WS Reference field for later retrieval with the tracking# for the import to history process.
If the invoice gets created AFTER the tracking# then other options open up. Making use of UPS WS Reference field again, this time with the SO Order number in it, I created a mod to lookup and import the tracking numbers directly into SO Invoicing at the time the user enters a Sales Order#. Might be possible to import them into the Invoices from VI, but I never tried that.
Big advantage I found with this method is, with a little Crystal tweaking, being able to print the tracking numbers directly on the invoice.