Those kiosks are ok for me most of the time, but they do not accept media mail and they have restrictions on the minimum purchase (try buying a single stamp). Since there are no longer stamp vending machines, you have to wait in line to buy one stamp. If you want a book of stamps, well, step right up!
Annoying too is they do not offer parcel post on the first screen usually... you have to select "show me more options" -- I wonder how many suckers buy the Priority Mail when they don't really need it...
Last rant:
If I walk up to the clerk and say (very politely too) all in one sentence: "parcel post please, no insurance, no confirmation, no extras, just the cheapest please" they will always say "do you want insurance?" "do you want confirmation?" "for just $2 more you can mail it priority!" etc etc etc
It's like talking to a wall...