Yes, you enter your credit card at time of purchase. But it definitely seems secure and I didn't have any issues (nor have I seen anyone post anything on any of their social channels that I follow about any issues). Hacking is obviously always a worry, but I think PhotoSquared is as secure as any other site/app you would order a product through.

As for the material, the squares are 1/2" thick foam core boards. They weigh next to nothing and (imo) look much better than the Mixtiles boards bc they are thinner and look less bulky/blocky. They come with double sided adhesives that let you stick them very securely to the wall. The printed surface is some sort of matte paper finish (so it isn't glossy like standard photos), but I found this works better for where I hang them bc the sun isn't causing giant glares off them. Mixtiles is the same surface (I believe). Like I said, for my money, Photosquared was def the way to go.

Yes. App is free, just pay for the photos you order. No accounts (but it was nice that they saved my shipping and payment info for my 2nd order). The shipping was also free for PhotoSquared and they arrived really quickly (like 4 days). Good luck!

Mixtiles is fine, but for my money, I would go with PhotoSquared. I've used both, and they are very similar products (same size squares, PhotoSquared is just a little thinner). But PhotoSquared is less expensive. 4 squares for $44 (vs Mixtiles at 3 for $49). Plus they ship really fast from PhotoSquared, and they definitely weren't asking me for any usernames or passwords. Worth checking out! Good luck.