Hi bhatnajam - I think you might be waiting awhile - I am still waiting a direct question to API support - and have even had the MYOB survey ask me how MYOB faired in answering my last question.
hmm - notwithstanding - often API support thought helpful when they reply often don't exactly know what their endpoints do exactly.
I know timebilling has had a bug in formatting an invoice with an inventory item included - it reverted to item invoice - and the only way around it was to always put the 1st line of the invoice as an inventory item - then it behaved.
My point is - I have written code to post most invoice types - but I can't give you an answer directly on your question - however, what I found in writing endpoint code is trial and error gets you up the mount a lot faster than asking API support.
Write a simple timebilling invoice post - 1 line and get it accepted.
Then try a simple time billing with an inventory item - if it isn't going to work it will tell you - however, the API error codes are not always helpful.
The Doc