Yes, you difently need a digital scale, you can find one on ebay for under 20 bucks, I think I paid 14 for mine and that was including shipping and mine is a 50 pound scale.
Then when you calculate your postage, make sure you choose a zip code that is the farthest from you. I'm in Oregon and use a Mississippi zip code to estimate all my shipping, that way I will never be under but can be over sometimes in which I just give the buyer a better deal on shipping when I invoice them.
Don't forget to weigh the item in whatever your going to ship it in and the materials. That's where I messed up when I started selling, always forgot to add extra for the box. Also, make Good use of Priority flat rate evenlopes and boxes. You can get those shipped to your door for free.
Ignore me if you already knew all this, lol.