Kyle's right. The variations in print color or density from Costco is likely a problem at their end, not yours. Sounds like the operator is not paying attention to the special lighting conditions of some of your pictures.

A quarter century ago, when I ran KIS-Photo and Noritzu processors and printers at a number of photo labs, we had to watch out for images that were exposed differently from all the others. Get your color and density all dialed-in on shots that are representative of the majority, and then print away. But when you reach a picture of a sunset or a nighttime scene or a train in evening glint that's up against a dark hillside or sky, and you'll end up with a very washed-out print if you don't compensate. The average person/machine doesn't realize that the vast areas of darkness are SUPPOSED to be printed dark, or even black, in order for the real subject matter to have proper exposure.

If you have challenging images to be printed at Costco or elsewhere, you can either bring them your own printout as a density guide, or simply explain to them your special needs. Not exactly the same circumstances as printing from negatives, but printing from digital files does require some of the same hands-on attention to "exposure."