Thinking about the volume of your mailing is key here.
For regular, non-EDDM retail rates, postcards under a certain size (see "Postcard Size") are charged as first-class mail, which starts at 35 cents per piece. Postcards above the technically-correct postcard size are charged as "flats", or large envelopes. See "EDDM Pricing".
It's helpful to remember that the scale of your campaign can drive down price per piece of mail drastically. Always keep this in mind when planning direct mail campaigns. https://pe.usps.com/BusinessMail101/Index?ViewName=Prices