We love the USPS and their flat rate program. We will use this whenever possible. That said there are several items that cannot go priority mail and they include: Ammo, Smokeless Powder, Primers, Primed Brass and anything over 50lbs. We use USPS, FEDEX and UPS and have learned that each one is great in some areas and terribile in others so there is no one best solution for shipping. The only thing that seems consistent is that your experience with USPS will be based on your mail carrier (mailman for us older folks) and their committment to doing a good job.
Our OVERALL experience since beginning online sales has been best with UPS if that helps.