9
Because the cost of postage is up until and include 1 oz.
10
Because this next price of $1.10 only is used if you are over 1 oz in weight.
11.
Because you can't mail something that doesn't have any weight at all. If there's no weight, there's no envelope.
12.
No. It wouldn't pass the vertical line test. The function would indicate two different prices for the same weight, which doesn't work.