No, the post office does not deliver within 3 days maximum, for first class.
It has been taking as long as 5 work days for some of our first class mail to be delivered.
We had a package shipped by priority mail on the 18th of this month, from New Mexico. It arrived here on the 23rd of this month. Five days later. It looks like the post office is having a hard time trying to make deliveries on time.
If your going to use the post office, I would make sure the plants have plenty of dampness to keep them from drying out.
Bob, zone 5 Connecticut