@ecomsilio I finally got it working! 
(the odds were a bit stacked against me today too… had to create a new US1 account to work on it)
So, the solution in the end was quite simple.
I created a new aggregate JSON module after the iterator that gets the transactions from the Etsy array. Data structure set as collection that replicates the properties needed for the HTTP payload.
Then, created a set variable module with a substring function to remove the enclosing [ ] that Make appends to iterator bundles.
Code
{{substring(JSON String; 1; length(JSON String) - 1)}}
Code referenced from this topic
After that, it was a case of creating the correct JSON format in the HTTP module’s payload for this variable + the shipping item, hit run, and boom!
Invoice created! Great success!
I then decided to remove the set variable module and just add the substring function directly into the HTTP content payload (no need to expense +1 operation when it’s not required)
The format ain’t pretty, but it works
I’ll experiment with another parse JSON and/or the existing aggregate JSON structure to see if I can output the shipping item and then transform it into the correct format to be added to the items array in the HTTP payload.
Thanks again @ecomsilio for the help & encouragement! 
Much appreciated 