I'll bet they use a graduated shipping system where when you go over $10 but under $15 or $20 it adds 2 bucks, and if your total is over $20 but less than $30 it will add another couple of bucks, etc.

I'd be willing to bet that you could add another pack or two of labels and the shipping charge would stay the same - unless your order exceeds $30 or whatever the threshold is for the next shipping charge increase.

I've done that with Numrich before - just kept adding small, low-cost items to my cart until the shipping took another jump - just to see what the thresholds were.