In case someone might find this useful, I went back to my post office with the Sepomex (Mexican postal service) rates for small packages I had printed from their website. These were the rates the same post office kept telling me that they were unavailable for items other than books, cds, etc even though the website doesn't mention this restriction. When I showed them the printed rates & asked if they were still current & if I could use them for any small package, I was told yes. This was the same officer who had told me about that non-existant restriction a number of times before. I was able to send a small package that day (as a test) which weighed 500g & cost me 135 pesos to Australia. Previously, they would have told me I could only send it with Mexpost which would have cost approx. 450 pesos.
The main restriction with this service is that the weight limit is 2 kgs & then Mexpost has to be used for anything over that.
I think post offices dislike dealing with this service & direct people to Mexpost instead but I don't know why. I was told by 2 different post offices about that restriction which is not true. I was given at least 20 stamps for my little package! Perhaps that's why they prefer not to use it? I read somwhere that the government wants post offices to use that service for small packages but they didn't say why they weren't doing it anyway.