Without further ado, here is the skinny on postage rates and regulations on postcards:
4" x 6" or 4.25" x 6" postcards
These mail at a first class individual stamped rate of 27 cents each. The post office has a specific stamp for these postcards. However, if you were to do a minimum amount of 500 and add a barcode (which qualifies your mailed piece as an automated first class item) you save another 5 pennies per card on the postage as the rate then is approximately 22 cents. In order for a direct mail imprint to be applied to your mail piece the post office has a requirement of a 2" x 4" section being completely white at the bottom right on the back of your postcard. That way they can feed it through their automated scanners to sort which saves them manpower costs and they pass that on to you.
5.5" x 8.5", 5" x 7", 6" x 9" or 6" x 11" postcards
These sizes all mail at a first class letter rate of 44 cents a piece if you are doing them one at a time with stamps. Here again you can automate them (still mailing at first class which delivers in 3 to 5 business days) by leaving a 2" x 4" section on the bottom right of the mailer panel completely clear of all type and graphics for address imprinting with a barcode and your rate comes down to around 37 cents a piece.
On larger items like this you can also do a standard rate mailing (used to be known as bulk mail). Delivery times on this mailing rate are officially stated by the USPS at 5 to 15 business days. Again it has to be a minimum of 200 pieces to qualify and there is a certain amount of sorting, labeling trays (or USPS supplied sacks) with the various ADC codes which allows them to sort to zones and then to mail carrier routes. The USPS has downloadable documentation that needs to be completed and you are required to deliver the pieces to a BMEU (Business Mail Entry Unit)
Most large printing companies (like us!) have in-house mailing facilities and are equipped with all the necessary postage indicias and permits for the different types of mailing classes, which is part of what is covered by a mail preparation fee.
The mail prep fee also includes retrieving, processing and setting up the data base for list, CASS certification of the list (a USPS requirement), zip+4 coding, ink-jetting addresses, barcoding (where applicable) sort-bundle-sack or tray, preparation of all postal paper work, and delivery to post office.
In order to insure efficient and correct delivery of first class, presorted, barcoded mailings, the post office has licensed certain companies to check mailing lists against their "master list". This service is known as NCOALink. As over 40 million people move each year, this service will update your list with both individual/family changes of address, as well as businesses.
While much of this information belongs to the technicalities of preparing a mail piece, being aware of some of the ways to save money (i.e. the 2" x 4" open area on the bottom right) obviously benefits your bottom line in the long run, and makes mailing a worry-free activity.