With summer in full swing, tourists are flocking to big cities for vacations in larger numbers, as are college students who managed to land internships in the city. This means you might want to change your target audience slightly to include tourists.
One ideal way to target the influx of people is by utilizing a postcard printing service and making your postcards available in several tourist heavy locations. For instance, if you live in New York City, try making a deal with one of several tourist locations, like Ellis Island or Carnegie Hall, where you pay them to sell your postcards or let you place them as free items in the lobby.
You could also design custom posters advertising your new postcards, with a number to text or a website to visit where tourists can get postcards for a fee or for free. Postcards are portable, attractive souvenirs, and are a perfect way to target the summer influx of tourists.