Upload & Sell: Off
I tried several services: Canvas on Demand was good with colors, but did not do good job stretching. Costco was OK with colors, and good with stretching. Besides, they print panoramic. Don't know who is their subcontructor, but prints came un-varnished.
Picture It On Canvas use very good frames for stretching, and colors are OK, but looks like they do not read instructions and applied extra processing to the pictures even if I asked not to. One picture is already showing signs of wear on the corners.
There is also Gallery Direct whom I did not use for canvas, but ordered metal prints from them, and could not be happier. Customer service is great, too.
All of the above have regular sales, and some deals can be found on Groupon or via Facebook. Prices are about $40-50 for 16x20 to 18x24 print. For comparison - paid $160 for 30x20 custom canvas print in local shop, and it was of outstanding quality.