not at all concerned!
I'm in the same boat.
Realistically, the LoA is the exception not the rule. The majority of appointed candidates did not get an LoA first.
They're worthless, too, with a medical DQ, a 12:00 mile, etc. You should NOT be worried at all that you have not received one yet as there is still time and you may open your mailbox tomorrow with a pleasant surprise. Even if you don't, it's the appointment that has value.
Relax, it's mid-October.
The hardest things we candidates need to learn with this process is to hurry up and wait, as the experienced posters say.