small letter is 250gm
large letter is 500gm
… I thought this was a seperate bug but maybe it is the same one being discussed here Australia Post real time rates for letter doesn't return rate - #5 by chickentwisty
For it being the same bug is same error message
Against it being the same bug is that the other letter description (per OP) works ok
Possible theory, maybe this particular option is now an unsupported option given that small and large letters with weights are listed in addition to this option that isn’t working?