I was actually thinking of using them with a similar promotion a few weeks ago (16x20 for $10), but decided not to because of shipping. Normally it would still have been worth it, but since Costco has $10 off canvas prints until 8/30 it ended up being cheaper to buy from them.
Costco usually charges $38.99 for 16x20 and no shipping (I believe it's in-store pickup only). With the $10 off it was $28.99 making it cheaper than the $29.99 I would have had to pay. I don't know how the quality of Canvaspeople's frames are, but Costco's are solid and feel and appear to be high quality.
I ended up getting 1 16x20 and 3 12x16 (normally $28.99 before the $10 off) and they all came out beautifully.