I know that Walgreens and Walmart's photo processing does an "Auto Enhancing" step that adjusts both color and contrast to "optimize" images. They do so on all images unless told to do otherwise. The Auto Enhancing seems to work well on things like point-and-shoot
images. Most customers don't notice the adjustment, and think the
prints look great when processed at that particular retailer. Unfortunately, it really screws with people who do their own adjustments at home. It's likely Costco does something similar. Therefore, whenever you have your images printed by a chain retailer, you must contact them and ask them to turn off all auto corrections.


However, even with all their auto corrections off, what you see on your screen won't exactly look like what gets printed. However, it'll give you a base to work from so that you can make small adjustments on your monitor to mimic the retail printing and, therefore, achieve results that are more predictable and consistent.