I've used both Zazzle
. Like other online printers, they are great for reproducing vector art ("graphics"). However, good reproductions of images with greater color gamut (photographs, scanned images, photos of paintings, etc.) can be iffy.
I've also used the local Fedex (used to be Kinkos) store that has a "print shop" inside and found the same thing, so would hazard a guess that other businesses with "print shops" inside pretty much run the same way quality-wise.
My suggestion would be to find a shop that will provide proofs of the calendars, not
digital image images of them, but paper copies. One will have to pay for them, but a shop looking for a long term client will often deduct that from the total cost of the order.