Hey, I wouldn't worry about it too much...yet.
I've done about 100+ deals on these boards and I've had my fair share of getting things out late. There's just some things that you can't account for.
All it takes is for you to get off of work late one day, have to pick up a kid the next, and forget to grab an address on third. Three very unintentional things and boom, you are almost half a week behind in shipping things out.
If the person you are dealing with is like me, I've got a 20 minute window that I can get to the post office. All it takes is one bad phone call before I leave work that afternoon and I've lost that window.
As long as he's in contact w/ you, I wouldn't get too worked up.