If you shipped in the late afternoon, it probably didn't get shipped until Sept. 7. It's only been a week! Shipping can take 7-10 days for USPS mail. Honestly, I'd just give it a few more days. If you haven't already, consider writing in your policies that orders within the US can take 1-2 weeks to arrive, because that's the normal time frame. Some orders arrive in 3 days, others take 13 days.
In the future, I would recommend using PayPal for shipping labels. You just to go PayPal, click all the things it asks you, print out the label, stick or tape it on, and send it! The good thing about PayPal is that the labels have a barcode which tracks shipping. That way, you can go online and see that an order has indeed been shipped or delivered, and your buyer can also see it on their PayPal. The delivery confirmation adds 19 cents to the shipping cost, but for me, it is totally worth it to help avoid situations like this.