then you must purchase a MPP cancellation device
This is not a requirement.
You will need a rubbert stamp that so states "Precancel Use Permit, etc.
This is not a requirement either, although it's not a bad idea. I have and use such a rubber stamp. What is required is that the mailpiece be endorsed with the type of service being requested, e.g. First Class. I use a rubber stamp purchased at Staples for this.
The other caveat is that you CANNOT place such mail in collection boxes; they must be presented to the clerk at the post office window where you hold the permit.
This is true. However, I frequently mail items franked with precancels with other mail. In many years of doing this, only two or three have been returned to me. In order to minimize the chance of an item being returned, whether mailed at the post office or elsewhere, I always include one low value tagged stamp in the franking. With the current 49c First Class rate, I'm using old 4c Steam Carriage Transportation coil scrap along with 45c in service inscribed stamps. Although newly issued low value stamps are no longer tagged, older tagged ones are easily found for face value or less at stamp shows.