The cards have a nice "pebbly" texture to them that's pleasing to the touch and look like a higher quality of card than your average postcard. The samples came with an FAQ about them. They're a little thinner than most cardstock, but very sturdy. It's recommended you use ballpoints, fountain pens or Sharpies, but not gel pens or fiber tips.
The images are bright and colorful, and the cards fit the standard 4x6" (aka 10x15cm) that many Postcrossers like.
You can buy cards as singles or in sets. Cards are 65 cents each, which is comparable to most IRL venues, and there's free shipping if you order more than $25 worth. It's a small family business - I got a personal thank-you note for my first order from Christopher Arndt himself. The postcards are printed in small runs and they're working on adding new stuff all the time.
(No I'm not paid to type all this - I'm just really, super happy with the cards I got!)
The sets I got are Trains (12 cards) and Route 66 (32 cards) because I see a lot of requests for those on Postcrossing in recent months.
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