If you have the Amazon FBA channel hooked up through Shopify it won't let you fulfill international, so that takes care of the Amazon part.
For Oberlo, I'm assuming right now you submit the fulfillment manually to Oberlo and then put the tracking into the order?
If that's the case I'm pretty sure I could automate that with my app Order Automator.
Here's how I'm seeing that play out:
1) If it's domestic and product is set to fulfill by amazon_marketplace_web, then it will be auto fulfilled to Amazon
2) For certain product(s), if shipping address is international, then I can make the app request fulfillment via Oberlo
3) If Oberlo is capable of updating the order, then it should send the tracking back when it gets fulfilled
That might work. If it does it would completely automate that process, which would be sweet!
I haven't used Oberlo in a few years, and I haven't tried to change the fulfillment channel on a product, so I don't know if this is 100% doable but in theory it should be.
If you're interested in that solution send me a message to firstname.lastname@example.org and I'll investigate it further.