No screen shot
To see for yourself what a confusing and unnecessary step it is to add a canvas to your workflow, set up a smaller version, say on 8.5x11 paper with canvas the same size as the paper. Select a color for the canvas so it becomes visible. Then print, with and without "scale to fit media" checked.
With scaling to fit media, you will see an unprinted small border around the canvas, which represents your printer's minimum border size, and the image itself inside the canvas will be reduced in size relative to original image sizing. Next, print with scaling unchecked. It should still have the same white border, as it can't print the canvas to the edge, but the canvas will be narrower than the first, with the image within the same size as you set up. Here it simply crops the canvas to preserve 100% image scale, whereas in the first case, scaling preserves all the canvas/image, but shrinks it to fit inside the minimum borders.
The utility of the "canvas" option is simply to activate an area you designate outside the image for a printed stroke around the image, colored border, or other manipulations, as long as it doesn't extend past the printer's minimum border width, which is where your problem started, it seems. It has no role in sizing borders at all, which you established with the relationship of image to paper size, with centered image.