i like flat rate because i hate standing in line. i don't yet own a postage scale. using flat rate, i can easily purchase accurate postage online 24/7. when i want to send a package that won't fit into my mailbox i just leave a note for the mailman letting him know where he can find it. they have picked up every package for me except one. it was raining buckets and miserably cold on some days.
i know i spend extra $$$ sometimes but my time is also valuable.
when pennies count more than minutes, though, i suck it up and go to the PO in person ... just in case.