My prints from Sams were attrocious, is it me or them?

A typical uncropped DSLR file would be of a 2:3 aspect ratio, which would yield a 4x6 or an 8x12 with no problem, but using that same file for an 8x10 (4:5) would yield odd crop results if left for the print vendor's software to decide.

If I were having prints made of the same image in various sizes that also happened to be different aspect ratios, I'd export a version of the file out of my editing app of choice for each aspect ratio so that I could control the crop myself ahead of time.