No offense intended, but how much experience do you have with shipping? The USPS Flat Rate boxes are too expensive for what they offer, and are really only worth the cost if you make them heavy. Like, lead-heavy practically - and it's generally tricky finding items that are heavy yet compact enough to fit in a Flat Rate box. It's almost always cheaper to ship using Priority Mail (or Parcel Post) in your own box.
And as Justin said, using FedEx Ground is cheaper than USPS Priority Mail on heavy items, usually by at least a few dollars or more. On top of that, FedEx offers automatic free insurance up to $100, along with full tracking. USPS doesn't offer tracking. One of the other pros of using FedEx is that I usually find that there's almost never a line; on the other hand there's almost always a long line at the post office.
I have enough experience with it. Fedex 2-3 day ground costs a little more than USPS medium flat-rate. If you want to make it faster than that, it costs way more. If the Audez'es are too big for a medium flat rate box, then I'll use Fedex to save myself 3 dollars. You don't have to worry about a line at the post office if you generate your flat-rate sticker online. Free tracking? Please. Tracking on a 2-3 day shipment is for people with too much time on their hands sitting at work in front of their computer who are too anxious to get their item. They're probably watching unboxings of their item on youtube on their other monitor as well.