I got the smart weigh High Precision Milligram scale from Amazon: https://www.amazon.com/Precision-Milligram-1-7637oz-Capacity-Calibration/dp/B072PQ5XC5/ref=sr_1_2_sspa?keywords=smart+weigh&qid=1580373585&s=home-garden&sr=1-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExSENEWkNaV1VURjFDJmVuY3J5cHRlZElkPUEwMzI3NDA3M05NRFpaUDEwNThITSZlbmNyeXB0ZWRBZElkPUEwOTY4NzAyMlA1NjRHRk4zU0ZOJndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==#customerReviews
This hasn't given me any problems and is dead accurate.
I searched for a scale that could measure .1mg back awhile ago, and didn’t succeed (never mind why I wanted one:) I’ve never seen a scale, short of an analytical balance (lab scales) that measure past 3 decimals. Can I ask why you need such precision for postage? A thousandth of an ounce (or if that scale measures grams, .001 gram) is a very tiny amount. I can’t imagine postage going up much when rounding to the nearest .001 ounce.
Edit:
After searching for a couple sec, I found this, though I’m not sure this is what you are looking for either.
Good luck.