ACL has a note on their homepage now saying that they sent the checks out on 9/28 and believe that there was a USPS delay:
Hi Shopper!
We have received numerous emails and calls regarding the status of August’s shop reimbursements.
We can confirm that the checks were mailed out on Monday, 9/28/15, as scheduled. It appears that USPS is experiencing delay in delivering the checks. We have been informed by some shoppers that they have received their check over the weekend and in today’s mail. Therefore, we assume that you should receive your check soon. Thank you for your patience. If you still haven’t received your check by 10/16, contact us and let us know so we can research.
Thank you!
Shopper in California's Bay Area