In my post (the OP) I just focused on the new box and left out some important and obvious conclusions/recommendations. To wit:
1. If the entire parcel weighs 13 ounces or less, best to send it first class parcel rate ($1.17 for one ounce; up to $3.21 for 13 ounces). 13 ounces is the maximum weight allowed for first class parcels.
2. If the parcel will weigh a pound or less, you can use your own packaging (with Priority Mail stickers or written words) to send it Priority Mail, for $4.80.
3. If you don't have a scale and think it is over 13 ounces, or if you know it is over one pound, then use the new small flat rate Priority Mail box, which can accomodate any weight that fits into the box and costs a flat $4.95 retail.

Robert