My Costco experience with digital images are that they are not cropped in any
significant way. It is so small of an amount that you can consider them full
frame in relation to the original file, so long as the file and print size
aspect ratios match. How much did they crop?

Generally they will whack off a couple pixels but it shouldn't be much
worse than that.  Various Costco stores have different machines,
however, and some of the Noritsus can crop severely when given images
in excess of either 320 or 400 dpi, depending on the particular model.

Anyway, I've had Costco process lots of 4X6  images for me, and
they've never done any significant cropping.  The machines in my
locality are all Frontiers (and also at Sam's), and these should
ideally be fed 300 dpi images.  I always specify that no adjustments
should be made, since I always provide optimal (1800X1200 px) image
size converted to an ICC profile specific to the Frontier printer
used.

BTW I see today that Sam's is doing 4X6 prints for 0.14 each.  Hell,
the paper costs almost that much.

It must be because their printer cannot handle images to be printed at
432 pixels/inch as you sent them.
Either resample your image to 1536 x 1024 in your photo editor or order
an 8 x12 print. If  you have never had an 8 x 10 (12) enlargement from a
5 MP camera, you are in for a treat.
Bob Williams

This is a problem when printing at many stores and booths.  Their
machines are very automated, and do not allow much leeway on how they
print.  This is NOT an aspect ratio problem, it is a problem in physical
size, or PPI.  Their machine is not set up to print borderless.

This is one advantage of printing at home, you can print to whatever
size you want, whatever ppi you want, etc.

Do not be afraid to trim edges with knife, sissors, or paper trimmer.
Nothing wrong with trimming prints, people have been doing it for a
century or more.