Also, I keep track of the weight of my fabric when I send out fabric.
1yd of cotton fabric (on average) weighs about 6 ounces.
I've had to put out lots of money in shipping costs, because I wasn't charging enough, or I decided to add delivery confirmation to an order, just to be sure a large order arrived, or could be tracked if it didn't arrive.
When I buy things from others, I hate being overcharged for shipping, so I guess that's why I try so hard not to overcharge.
It's a learning curve...I think I'm finally in the ball-park of collecting the right amount for postage now.