I don't think it's a typo. I think the police saw the receipt dated 7/3 for the duct tape when they searched JRF's trailer. Then they went to the PX and checked out the security footage from that date.I wonder if it was a typo, since the investigation seems to have really gotten going (warrants and requests for surveillance) in that first week of August.
I'm very curious about the point of that, otherwise, as well. Was duct tape the only thing he bought? We probably have to wait quite a while to know the answer.
Obviously duct tape is one of those items that sets off an investigator's radar, but it could very well just be a coincidence. The other man seen with JRF left with the tape, so JRF could have just been doing him a favor to get discounted pricing.