Hi,
I am unaware of the intricacies of the Amazon Buy label through Seller Central. I can confirm that through the Ordoro API, order items are sent directly to the Amazon Buy Shipping API for each label request.
If you wanted to ship one line item with one label, and another line item with another label, I would expect this to create two labels, and associate the labels with the line items in Seller Central.
Edit: And to separate lines like this in Ordoro, you would need to split the order.
Hope that helps,
Sophie