Hi I would do as others have suggested and get the guage. In my opinion the pacifers that you are selling I don't think will lbe able to be sent as a large letter. You  have to be careful as Australia post will charge you the  extra post plus a fee and it might be nearly the cost of what you are selling. Plus sending them with no protection, I would be worried about them getting damaged by the machine and/or other mail on them.

This is just my opinion and I wish you all the best and good luck with your future sales