Costco typically uses Fuji Frontier printers which have specific dimensions for best output. If you don't use these exact pixel dimensions, the picture may be resized which will result in a slight softening. You can get better results by resizing, cropping, and sharpening with your own software.
The sizes are nominally 300 DPI with a little bit of overscan. From Dry Creek Photo:
Paper (in) Paper (mm) Canvas (px)
4 102 1228
5 127 1524
6 152 1818
7 178 2138
8 203 2434
10 254 3036
12 305 3638
15 381 4536
18 457 5440
For example, to make an 8x10 print you want to resize your image to 2434x3036 pixels.
Also be sure to specify that you want image enhancement turned OFF so that the contrast and colors turn out the way you intended. Make sure the image is saved in sRGB.
One last thing, load the files directly at the printer location, don't do it over the web. The web software may attempt to shrink the file before downloading in order to improve the load times; that will definitely have a negative impact on your files.
And to preclude the nitpicking, these printers are based on a photographic process, so DPI = PPI.