Sometimes the letter or package is stuck somewhere in the US mail systems.
E.g: whenever I rent a vacation place in little hamlets in France, I send my cheque by regular snail mail - after 4 days the owner will e-mail me telling me that he/she has received my payment and that the signed contract is in the mail - delivery to Montreal 4 to 7 days - dependin from where it is coming and the time it was mailed.
This summer, I did the same for a VRBO rental in Maine. After 2 weeks, the owner has not received my letter enclosing my cheque. I went to the bank to cancel the first cheque and resend a second one again through registered mail with a tracking number. The letter left Canada within 2 days, reached the border on the 3rd day and then silence. The first and second letter reached the owner on the 4th week the first letter went out of Montreal.
So it must be sitting somewhere ....( I was told that sometimes even a letter is set aside to be "scrutinized" by an agent from some agency)