For the cards, there are plenty of options and you can weed out the ones you pay for if you don't have two coins to pay for a card. Even then, there are about 50-70 free designs available, so it shouldn't be hard to find one that fits your needs. There's also the option to upload your own design. You could totally put your book cover on there! You can also add an additional photo or blank card for no extra coins.
This is noteworthy because to send the card to someone is one coin per person. Each time you choose something that costs a coin, the price is per person. So if you have a card and use an envelope with a background that's two coins, and if you send that to five people that is ten coins. In short, coins add up pretty quickly if you're not careful. But, you can send a card completely free. It's just if you want to get fancy, you have to use coins.
Would I use it?
It's a solid site, but I'm not entirely sure this fills a need i have. Either way, good luck to them, and thanks for letting me play around the site and give my opinion.