I can only advise based on how I received my 'v' visa.
I had to go to the American Embassy for a medical and the v visa interview. Once they were satisfied that I should receive the v visa, the visa was then glued into my passport with an issue date and expiration date (ten years from the issue date). When you arrive in the States you receive an I94 which is valid for two years and is obviously renewable when the two years expires. If you leave the states for a short period during this time, you must hand in the I94 when you leave and when you return, you receive a new I94 valid for two years
It seems odd that yours was posted to you based on how I received mine (in person), but then again maybe there are different situations.
Hope all goes well.