Updated: Dec 17, 2021
Well, the good news is that my Christmas cards arrived. The bad news is that they've printed so dark that you can't see what's going on in the pictures unless you hold them under a very bright light. I've lodged a complaint with moo.com and hope they reply in time for me to either order some new cards or buy some off the shelf to send to family and friends.
I'm open to the possibility that the way my pictures look on my computer isn't how they look when printed, but then there should be something clear on the moo.com website saying "turn the brightness down on your computer monitor while designing your cards". Also, is there nobody at their end who looks at the image and says "this looks wrong?" or is their whole workflow automated?
If the worst comes to the worst, I'm going to just have to print the images myself and stick them over the ones on the cards X(