Has anybody used Costasur.com to make any sort of booking in southern Spain? Or does anybody know if it is a reputable web site?

I'm trying to get the Renfe website to actually sell me a train ticket, but I'm not having any luck. I noticed that Costasur.com is able to book Renfe tickets (for only about 5 euro more than the Renfe site) and am wondering if it would be safe to book through them.

Any advice/comments are much appreciated!

Thanks!