UPDATE :
I just checked my balance and the 4th envelope I received was credited to my account with the correct number of credits. (4 credits for 16 wendy's coupons).
It is possible that the sorting machines could have damaged the envelope, but I really think something mischievous happened.
At least i'm not as mad as I was a few days ago.
I should still file a complaint with the post office. I need to get the details from my dad as to exactly where he mailed it from.
Next time for something like this, I definitely need to do registered mail. (eventhough I noticed the last poster didn't have luck with that).
Thanks everyone.