I go to your local Post Office. Most of them have a lobby that's open 24 hours for post office box users. And, there's a really nice scale typically available 24 hours. The scale they have is super accurate and according to our post master, its re calibrated every day. I started using it for Pinewood Derby cars (max 5 oz!) and found it to be dead on.

I've gone a couple of times during the day too and the clerk has come over and helped just because he wanted to see my truck up close. A few weird looks from customers too during the day, but I can handle that.