As suspected, Costco tends to resize the photos that you submit, so the artwork came out a little too big. I pulled out the micrometer, measured it, compared it to what size I want it to be, then resized my master accordingly.
I have re-uploaded the artwork to costco and ordered a print. It will be ready for pick up tomorrow.
The good news is that the print looks REALLY good. I'm very impressed with the print quality. If I can just get the size right, this will definitely be a great thing for the kit, and will make it that much better.