Any small scale should work. I begun with a cheapo kitchen scale. Anything that measures in ounces is worthwhile since First Class is tiered based on weight (4 oz, 8 oz, etc).
I've had a Dymo M10 (10 lb capacity) for a few years and it's been perfect for me. You could even get the M5 (5 lb capacity) if you're only dealing in small things. I have a Brecknell GP250 for large packages, but that isn't helpful for small items.
A scale is a worthwhile investment with postal prices the way they are.