The big box stores are usually a triple-win for cost, convenience, and quality, but there are some things to watch for. Do a preliminary scouting trip and look for:
- What machine are they using? Some of the stores have put in new models of photo printers that I don't quite trust. I prefer a machine that's new enough to be in good shape, but old enough to have been evaluated by the archival crowd. The Fuji Frontier machines are generally very good. Whatever they're using, look up light fastness test results on a site like http://www.aardenburg-imaging.com./
- What paper are they using? Extra points for Fuji Crystal Archive, though again there are many good and many poor options out there.
- How are they handling the prints? Is the tech wearing gloves? Are they super-careful not to bend the corners?
Before you print a big batch of photos, print a couple of test images. Print a really light details on a light background, a dark one, one with lots of blue sky, and one with big areas solid gray tone. Some stores (I don't know about Costco) calibrate their machines only once or twice a year. They may print very well after calibration, but eventually they can drift and produce not-so-good prints.
Finally, if you're keeping your archival copies on DVD or CD, keep in mind that there is a huge difference in longevity depending on the construction of the disc. Be prepared to pay for true archival quality with a gold reflective layer.