I have one of the thick cardboard measuring devices that the RM were giving out when this fiasco of a postage system came into force. I now measure and weigh my own items and if it doesn't go through the slot cleanly, I go up a size. I then buy my postage on line and so far (touch wood) it hasn't been wrong.

The only time I go to the PO now is if I have parcels or need a special delivery. One thing about print your own postage though, you must post it by the date they specify (usually the next day) or you have to pay again!