I ordered a 14x17 mat with a 10x13 opening to use with a 14x17 frame from the same seller. The mat is just as described and, as far as I can tell, great quality. It's probably better quality than I would have gotten at Michael's or Blick (where I first looked but decided against it because of the expense). One word of advice if you don't frame art often, definitely take advantage of their custom opening sizing. The document I want to frame was just shy of 10x13 and the seller reaches out to inform you that the mat will be cut 1/4 inch smaller (9.75x12.75) so that the document won't fall through. I didn't care enough to double check the size of the document because I knew it was "close enough". It's just barely fits because the document I have is probably 9.8x12.8. So I'm happy with it, I just have to be careful with assembly.
tldr: Seller gives a personal touch, great quality, great price, ships on time, take advantage of their custom cutting!