Reply #4: No need to "priority" mail it, but put it in a real envelope with a stamp.

[Karmageddon]

Put it on good bond paper (same as the resume she presumably gave to the interviewer). Just like you dress nice and comb your hair and try to look your best, the "thank you" letter is another view of you that the interviewer sees. You want every view to look good.

Besides, if it takes 2 or 3 days to get there, that's a reminder 2 or 3 days later, after who knows how many other interviews have occurred. That's a nice time for the interviewer to get a reminder of how classy, respectful, and considerate your wife is, how much she is attentive to the details, and how good of a representative of the company she'll be.

Just my two cents.