Our local post office has a hatch where you can put the bigger items too. No need to wait in the huge lines (longer lately due to the small branch closures).
Now I feel your pain Salazar, I really do, but the Post Office staff aren't monkeys. The training required to serve on the tills in a post office is extensive to say the least. Far more so than most till roles.
It is also part of the job to push additional financial products and 'add ons' (Stamps, Jiffy bags etc) to customers. Batteries in Maplin, Terrys Choc Orange in WH Smith etc etc. They are all at it these days. It is common sense of course to use discretion with these point of sale conversations in peak times (though this is now all day, every day in my local PO).