These are pretty good. They lock together well and are reasonably durable.
So are they all only corner pieces in the pack? That’s what it looks like in the pic
If you look at the 6th pic it shows there are border pieces that attach for the sides.
What are the dimensions of each mat?
What are the dimensions of each square?
It says in the description that they are 24"x24".
I just tried to add these to a new cart. I had ordered something ten minutes ago. The second cart is charging me $5 in shipping. I thought it was $5 to ship all day long. Or was that some old promo that I’m remembering? Or was that the old woot? You can get these for less on amazon unless you don’t have to pay $5 for shipping.
It’s $5 per order. We haven’t had $5 all day shipping for probably 4-5 years now. 
What’s the texture on the bottom side of the mats? The same, or more smooth?
Per the vendor, the bottom is flat.