This is mostly just a sanity check: Is the “Multiple Shipping Routes” feature not supported on credit memos and return authorizations? I ask because I see the “Enable Item Line Shipping” checkbox on credit memos exactly the same as on invoices, which is what I understand to enable that feature on a specific transaction. Also, I was able to import line-level shipping addresses onto the credit memo, which appear on the credit memo itself and would lead me to believe that it is supported. The issue comes in when Avalara is used to calculate taxes, but uses the shipping address from the first line for all lines, which gives us the wrong tax rates on most of the lines. The only reference to this specific problem is an Avalara help article from 2016, but that does seem to be backed up by NetSuite documentation.

I mean, what's the major advantage here of being able to do it on a single transaction?

Yes, Avalara’s generating the correct taxes on invoices with MSR turned on (“Enable Item Line Shipping” checked). The advantage is that we have weekly invoices for our customers that need dozens and sometimes hundreds of different locations (across hundreds/thousands of lines) with very different tax liabilities. Our customers prefer one invoice per week instead of one per location each week and if we need to credit one out, I’d rather not require Accounting to create dozens/hundreds of CMs against one invoice.