There are multiple takes on postcards.  Here are a few:

First class card – You can mail a piece that is no larger than 4.25”x6” and no smaller than 3.5”x5” for 34 cents as of the date of this writing.  Bulk rates are as low as 25.3 cents per card.  The minimum quantity for first class bulk mailing is 500 pieces.  Delivery normally takes 1-3 days.

Large postcards – Pieces up to 6.125”x11” are considered “letter mail”.  The full first class rate is 49 cents.  Bulk rates for first class mail can be as low as 37.3 cents.  Bulk rates for Standard Mail service, which takes a little longer to deliver and doesn’t include forwarding or return service, can vary from 28.8 cents down to as low as 15.5 cents each.  This postcard size does qualify for discounts if delivered to a local post office

“Jumbo” postcards – These are postcards that are too big to qualify for letter rates, typically 8.5”x11” or as large as 12”x15”, which is the maximum size.  These are termed “Flats” by the U.S.P.S. and postage rates can vary from $44.6 cents to 75 cents for first class mail.  For standard mail jumbo postcards postage can vary from 15.5 if sent to every address from a local post office to 55 cents.

A variety of factors can affect postal rates, and what is the best size postcard to use.  Postage is a large factor in deciding the card size but the questions below can help determine what will be the best postcard size for your direct mail campaign.

How many pieces are being mailed?

What is the geographic concentration of the recipients? The more concentrated the recipients, the lower the rates.

Which post office are you entering the mail at?  The closer you enter mail to the recipients, the lower the postage cost.  Postage savings have to be weighed against trucking costs.

Do you qualify for lower non-profit rates?

How quickly do you need the postcards delivered?

What is your budget for postage and printing?

How much area do you need for your copy and design, and how large do you want the piece to look in the mailbox?

By asking some of these questions you direct mail house can help determine what would be the best size for your next direct mail postcard campaign.