I made no mistake.
It's not just about surprise costs, it's also about personal responsibility as a seller. If it was your fault that it happened, why should the buyer be expected to deal with not just additional unforeseen costs, but likely deal with drama from the post office over it?
I guess the unclear detail is why there's two boxes at all for one thing. From the sounds of this "I-mind6", you were the one to box it both times, right?
You really don't want to see that this is your fault, do you? As a seller, isn't it your responsibility to post the shipping costs for the buyer to pay?
Your point made there doesn't nullify the whole "stealing from the government" thing. You should have bought two labels since you were sending two boxes, and you should have known it'd take two boxes before stating the shipping cost.