Haven't tried them but why print at 13x13 when you can just get a professionally printing 12x12 elsewhere?

I wholly recommend whcc.com - you don't need to order 10 copies.. single copies are $2.50 each and since you provide the file in the exact crop size (12x12), they don't crop. They print on professional Kodak archival paper with a wonderful lustre finish. I use them for all my professional printing and they are fantastic. Once you try them.. you'll wonder why you ever use a place like costco... Good Luck!