I do, and it's getting to the stage now where I wish I didn't.
The online system is giving some small discounts,presumably to encourage people to use the system. Unfortunately, the "real estate" post offices haven't been made aware so when they weigh your parcel it produces a slightly higher cost. They don't seem to realise that if you've paid online then the price is probably right. Instead they take great delight (it seems) in refusing to accept the parcel, even with the offer to pay the 5p or so extra for the sake of expediency.
Complaining to the Post Office customer services roduced the following answer:-
"Yes, we know about this. It is taking time to educate individual post offices. Take the parcel back and insist that they either accept it or contact us to confirm."
So you take it back to be told "Your computer must need updating, our systems are never wrong"
Only the Post Office...