I'm on a one year "Extension of Stay" based on business with Non-imm B and my wife has O as dependent

and baby is on my passport (I think this not relevant).

The questions is more a curiosity as lack of law rules but to save almost 4000 Thb (or save 6000 if they count also my baby)

to buy the 7 days extension and as well to be on the safe side with the law, it would be possible to present the letter of job/visa

cancellation to the immigration 1 week before your job will be officially terminated and leave the country the day after you present the cancellation letter to the immigration ?

Example : Monday go to immigration to cancel and Tue or Wed leave the country but job officially terminate on Friday.

As my boss will not mind to give me a week or so of holiday in my own country....let's say....

I'm aware the VISA will be automatically cancel as soon as you step out of the country (non having the Re-Entry permit), but do not want occur in problems as Mario2008 mentioned few times.

Believe letter is required to be actual last day of work. But you should be able to present the letter on Friday and leave Saturday without a problem as one day of overstay is not charged at airports.