Many cards, especially ones by artists such as Esther Ramirez of Essimar (Image 3), are works of art in their own right. Cards also offer access to art that otherwise might be prohibitively expensive: while I can't afford a Rob Ryan print, I didn't hesitate to buy one of his laser-cut cards for Roger la Borde.
None of the cards here costs more than $7 each, and if you use a standard size frame that means a minimal overall investment. If your card is an unusual size, you can try matting it yourself. Then, if you ever decide to switch out your art, there's minimal monetary guilt, and -bonus- you have a nice card to share.
Some of the cards highlighted here come in sets: perfect for you to keep a few and use the rest for actual correspondence.
1. Botanical card set by Rifle Paper Co., $16 for 8
2. Floral cards from Becca Stadtlander, $12 for 4
3. La Mano card by Esther Ramirez, $7
4. Paris cards by Darling Clementine (available as a set on Etsy, $35 for 12, and some individually through Little Otsu, $4.25 each)
5. Yosemite card by Claire Teschel, $4
6. I Heart You card by Blanca Gomez, $3.95
7. Letterpress Hello card by Sycamore Street Press, $4.50
8. Good Catch letterpress card by Egg Press, $4.50
9. Assorted Dog card set by Rifle Paper Co., $16 for 8 or $4 each
10. Framed Rob Ryan and Essimar cards in my bedroom
(Images as linked, 10. Leah Moss)