Using these?

You just use it and you get an icon on your character. That indicates you get bonus for one set of goods. The next time you pick up commerce goods, the icon will disappear, and you have to accept the prompt given in order to continue commercing.

The page details how the letter works, but if you're asking then are you confused with something the page states?

Wait, do I use it before purchasing the goods?

Yes.

Well now I feel stupid

I thought you only had to use it before selling your goods.

That works as well, the bonus of the "buff" is applied when selling at least one item. The Ducats you get on use will be received regardless of when you use it. It's like a Shadow Crystal, you can use it either before the mission or while you're already in it.