If the empty box and packing material = 280 g, then you are good to go with 900 cards mold chips. I actually get 19.99 lbs with 906 chips. Just saying you are getting so close, you would want to weigh the label, any tape you're going to use on the box, any dust that might fall onto the box on the way to the post office...
Also don't know how picky they are about the 20 pound limit. They may have a small allowable variance. Like what are they going to do if it's 20 pounds and 1 ounce? Send it back? confiscate it?