A letter to/for Jason?

What a great question! The answer is (1) A letter for Jason.

One writes a letter to someone. Having been written, the letter is a letter for that person. So the mail carrier would be at the door with a letter to Jason only if s/he had written said letter.

To take another example: The person who used to live in my apartment has moved to a different apartment down the hall. However, she has not updated her address with the post office. Consequently, a lot of letters for her end up being delivered to my place. I am considering writing a letter to her asking her to fill out a change-of-address form at the post office.

The difference lies in the role played by the person getting the letter. A letter is written to an addressee. It is intended for a recipient. For the person writing the letter, Jason is the addressee, so the person is writing to Jason. For anybody else involved in getting the letter into Jason's hand, Jason is the recipient, so from this point of view, the letter is for Jason.

Both are acceptable; there is little difference in meaning.

I would say that "It is a letter for Jason". The preposition used in these two sentences doesn't necessarily change the meaning.

The mail carrier most likely doesn't know what's in the letter, and neither do I; therefore, for us, both of these sentences are essentially the same.

Even though the letter is addressed to Jason, it might contain something intended for someone else. In this case, the preposition used changes the meaning based on a knowledge of the content of the letter.