When printing, you want to make sure that the option fit to page is not selected. It is selected by default. Whenever I would go into the stores, I would have to make sure to tell them not to fit to page. Essentially what that does is it allows the print to be printed at the measurements you select as opposed to what the computer thinks it should be.
Yes that's right... that is what you would do if you go in person. However, for me, my situation was different because before any possibility of being able to make sure that the option 'fit to page' is not selected, uploading the image to Staples automatically converts it before you ever get a chance. Once you do click the option to "none" instead of "fit to page" nothing changes, since the image has already been automatically altered. I found a work around. I simply enlarged the canvas in Photoshop to 13.5 inches by 7.7 inches to 'dupe' the uploading system into not altering my print. Then in the preview, I noticed it was a legal sized page to start with, indicating to me that the software didn't convert it. I went to Staples today to pick it up and voila! My theory was correct. It was printed perfectly... cut it up, put it in my UGC and boom, fits perfectly! So happy. It turns out that the Staples employees were indeed doing their job correctly & printing with the instructions I gave them (no resizing), but that damn software which automatically converts the jpg right after uploading it is a real pain. But hey, I found a work around now and am going to spread the word via this forum in case anyone has a problem.
So yeah, folks, if you upload your perfectly-sized images to Staple's website and you correctly set all the options, yet your image has shrunk, you can bet you were having the same problem as me. Again, simply enlarging the canvas tricks the uploading software to not shrink the image. This may only be a problem in Canada as we seem to use a different website than our American brethren... But yeah, good to have this info out there because my Google search never helped me!
Oh, and one more thing. When resizing your canvas size (not the image size!) make sure to use the Batch Processing function in Adobe Photoshop. It would be hell to do this monotonous task zillions of times... you must learn how to use Batch. Lots of info on the internet about that one!
