Chris - I think you setting the image to ‘fit the page’ may have caused the sizing issue (or could be an issue if Staples’ printer was set to this setting too?)

If the printer has a ‘scale setting’ feature (not all do) - make sure it is set to ‘off’ or at 100%.

I usually set the margins on images that I print to 0.5cm on all sides just be sure too 😃 (when I used to print covers I copy the image to MS Word and do a black & white draft quality print - just as a test print to ensure I it correctly sized)

If you have any independent printers or copy shops near you that do this kind of service I’d recommend you try them - in general, I find the people at independent print places will likely have more knowledge / experience than those at Staples.

Good luck with it mate - and let us know how you get on 😃