Apologies for chiming in so late. I have a lot of experience with this subject having a career as a graphic designer for 30+ years. I assume the photos of your paintings are excellent [high resolution, sharp lens, wide dynamic range [bits per pixel], evenly lit, white balanced, etc]. If not, your images will be problematic right from the start. Laptop & mobile device screens should never be used for color correction or to photograph your work with. Pick your best monitor that is not a CRT and calibrate it. Also, it would be extremely helpful to have Adobe Photoshop or Lightroom or Apple’s Aperture software. If you do, then you can assign a Color Profile to the image. This will embed code in the file that display monitors can read which helps to get homogeneous color accuracy across different screens and applications.
The calibration device you mentioned is overkill in my opinion. Unless you’re doing work that needs 4+ color (CMYK) ink process printing or very high-end digital prints like Giclée it isn’t worth it. If you have a fairly new LED or LCD monitor with high resolution and a wide color gamut & contrast ratio, obtaining decent color accuracy shouldn’t be problematic. That being said, it will be accurate only on your screen. It will not look exactly the same somewhere else. Research the specifications that Cafe Press requires. They should say something about what color profile to use, resolution, file type, etc. If your paintings are only for the screen (like here on DPW) then sRGB is the color profile to use.
Hope this helps.