When I go to complete an order my default carrier is Australia Post, which is correct.
But, as I am entering the tracking number, the Complete order button gets greyed out and this error message appears.
The number you entered isn't a valid UPS tracking number
I have to cut and copy the full tracking number, cancel out of the window and try again, this time by pasting in the tracking number instead of typing it.
This intermittent bug has been around for several years. It seems to hit when the AusPost tracking numbers have a certain pattern. Today it happened on all four of my orders. The Complete order button went grey after I had typed this partial string - R33965301435.
Security Note: this is only a partial string and cannot be used to identify any full tracking number.
