How is Kant's "Categorical Imperative" described in the situation of buying postal stamps?

I suppose while you are in the line to buy the stamps, it would be contrary to the imperative to knock over everybody else in the line in order to get to the front of the line.

Suppose I discovered that I could "slip one through" now and then. I could put a letter without a stamp into the box, and through habit it would be delivered, and I would save some money. Perhaps nobody at the post office would even notice I didn't put a stamp on. Or, more realistically, perhaps they'd ask the recipient to pay.

Either way, I might get away with something.

But, upon reflection as a Kantian, I come to understand that this isn't universalizable. If everybody did it, the postal system would collapse.

Also, I'm treating the postal carriers as means to my ends by fooling them in this way. Not as ends in themselves.

How's that?

Buying stamps is a non-ethical action and therefore the categorical imperative doesn't really apply.

Wouldn't you wish that everyone buy stamps to remain ethical?