The easiest way to use them is to go to usps.com and use there free click n ship service. By free, I mean you do not pay anything over the cost of postage. You pay online via your credit card. Then you can print out a label on your own computer's printer and attach it to your flat rate priority mail box and you are good to go. Just hand to a postal delivery person or drop off at the post office.
You can get free flat rate boxes delivered to you - visit usps.com to order them.
Flat rate boxes are just that - for domestic shipments it is one flat rate regardless of how heavy or where it is going. If it is fits in the box, and it is allowed to be shipping thru the US mail, it goes.
Priority Mail Flat Rate Envelope $4.90 12-1/2" x 9-1/2"
Priority Mail Small Flat Rate Box $4.95 8-5/8" x 5-3/8" x 1-5/8"
Priority Mail Medium Flat Rate Box (FRB1) $10.70 11" x 8-1/2" x 5-1/2"
Priority Mail Medium Flat Rate Box (FRB2) $10.70 13-5/8" x 11-7/8" x 3-3/8"
Priority Mail Large Flat Rate Box (Domestic Addresses) $14.50 12" x 12" x 5-1/2"
Priority Mail Large Flat Rate Box (APO/FPO Destinations) $12.50 12" x 12" x 5-1/2"