Upload & Sell: On
You need to be more concerned with the number of pixels in your file for printing and how far apart they're spread for a specific image size.
As has been explained, you have the choice of saving a file uncompressed, with lossless compression or with lossy compression.
The typical workflow for your type of situation would be to save an uncompressed 16 bit per channel tiff file from your raw file and make whatever changes to the file in terms of color, retouching, layering, etc. in Photoshop, save that, then make a duplicate copy of that file to upload to the printer and save that as a jpeg, usually a number 8 or higher. If you work entirely within Lightroom or Aperture, you would do whatever manipulations necessary there and then export a jpeg to be uploaded.
The size of a saved jpeg on disc depends on both the quality level you choose and image content. If you start with two images of the same pixel dimensions, one with a complicated busy background and the other with a pure white background, the white one will save as a much smaller file even with the same compression settings.
Tiff files also support zip and jpeg compression, although you should never use jpeg compression on a tiff as it sort of defeats the purpose of tiff. Zip compression, however is completely lossless and much more efficient than LZW compression resulting in much smaller files, but does take longer to save. Zip compression also works great with 16 bit per channel files while LZW often results in files that are actually larger than the uncompressed files (when the 16 bit files are actually 12 or 14 and not true 16).
While it's generally true that repeated saving as jpeg further degrades the file, you actually have to change at least one pixel value for that to happen. If you just dupe the file and save it with the same compression level, it will not degrade, but change one pixel from 243 to 244 or change the compression quality at all, then it will degrade.