Dave and all,
True you cannot use a USPS Priority Mail box to mail by any other means than Priority Mail. The boxes are free of charge to you and can only be used for priority mail shipments.
You can re-use the box for other Priority Mail shipments - I have done that many times when the box was usable.
If not usable, please recycle that cardboard.
In addition to the Flat Rate boxes that you can pick up at the post office, you can go on-line to USPS and order the 12x12x8 inch priority mail box at no cost. You can order either 10 or 25. I order in lots of 25 since I ship Priority Mail regularly.
Use USPS.com to check the Flat Rate vs. Regular shipping. For long distances and heavy objects, Flat Rate is better, but for near-in delivery and lightweight items, regular priority will be less expensive.
Keep your boxes to less than 12 inches in any direction to avoid the price uplift for large packages.
You are correct, the retail counter person at the post office is normally very helpful. I commonly wait in line while the counter person is assisting customers, explaining their options, etc. She rarely does that with me because she knows that I have been shipping USPS long enough that I know the "rules", and she can process my shipments quickly. I had 3 to ship today, and the process only took about 10 minutes - most of that was her entering the information into the shipping system - perhaps I should use "Click and Ship", but so far I have not and enjoy the banter with the folks at the small post office closest to me.
USPS Priority Mail is a great service and a good bargain. Only a bit more than UPS Ground and much less than UPS 3 day. Delivery is in 2 or 3 days. I have had only 2 minor reports of damage with USPS Priority since 2004, and I normally ship 60 to 100 parcels each year. That is a good percentage.