I'm using Illustrator and have upload many images with no problem. One of my latest images is centered when I save it in illustrator but then when it is uploaded to Cafepress it appears not centered on the products. Any ideas? I've tried resaving it 5 times but same results. I even tried their template. My image is not square but most of mine are not. Let me know what you think. Thank you!

Is your file PNG or JPG? What is the dimension in pixels? Is the image off center on all products or on particular products? Is the background transparent?

Silly question, but when you are working in Illustrator, is the entire window showing or are you accidentally centering it in only part of the actual canvas?

The easiest way to check is with it open, are there any slider bars to the right and bottom of the screen, indicating that the actual canvas you are working on is larger than what is showing on your screen?

It isn't just a matter of centering it in the window on your computer if the canvas is larger.

I don't work in Illustrator, but Photoshop has a "Fit on Window" command under the View Menu that resizes the whole canvas to show.

I have a few designs that, when mathematically centered, visually appear off-kilter because there is a lot more artwork on one side of the true middle than the other. I have to 'cheat' with these and, before I upload them, nudge them so they appear centered to the eye when in fact, if you measured them, they wouldn't be.

This phenomenon drove me crazy until I figured it out.

This is true. For instance, say you have a figure of a man with both hands down by his side it's a no brainer to center. His body mass would be center.
But if the same figure has just one arm pointing straight out to one side... you have to wing it since his shoulder would likely be the center.

Are you using Crop Marks in Illustrator?

Make a rectangle the EXACT size and position as your artboard. With the rectangle selected, under Edit(?) somewhere, is Crop Marks>Make. This turns the rectangle into a clipping mask and when you export your artwork (like into a jpg) only what's inside the crop area/clipping mask will export.
This way no clipping shapes outside your art area contribute to the size of the export.

Jim