I have no experience ordering canvas prints from discount sources, I print my own, but since no one has spoken up I thought I'd reply to your post. For most online print sources, you don't need to do anything special to prep your files beyond what you would do to print them at normal sizes. They will scale them to the right size and usually are happy with a high quality jpeg or tiff at native resolution (not uprez'ed).
If you are shooting for the best results you can get, then many photographers would work with a printing company where they can download a profile from the printer in order to soft proof their image first in order to customize the image to the actual printer characteristics. I would think if you are in that group, then you would be more concerned with what type of canvas and varnish and process the printer is using as there are many levels of quality and appearance to choose from. Lacking any real knowledge about what your print offer is offering, you might just have to try one as an experiment and then maybe try one from a well respected lab to compare the results.
You will find some images are great for canvas and some are better on paper. For example, canvas tends to be used more when high levels of fine detail are less important.
good luck,
Stuart