Hi there,
Indeed it acts quite strangely. Could you do some test for me?
For example, I would like to know which method is used and when, so could you try to name your shipping method differently like "air mail - 1", "air mail - 2" and check which one is used for each case?
Also, since you just have 50g products, why don't you set a wider range for your weight limitation? I mean, instead of a range from 101 to 150 you could set from 125 to 175 so you'll be sure two products ordered will be included in this shipping method (but this doesn't explain your problem and it may not fit your needs).