Mel Bay works several ways. They might accept anything between "finished product" and handwritten manuscript. If the former (which it sounds like what you want to do), you need to make sure you're using a program which they use and can print from. You'll need to use their page format, margin requirements, all that sort of stuff.

Or you could send them a handwritten manuscript, and they would then either set it in-house, or more likely ship it out to a freelance "engraver" (that's the term still used, though there hasn't been any actual engraving for a good while) such as myself, who would typeset the music and text if needed (though it's more likely you could send that as a text file), assemble the book, and send them th finished product.

I'd imagine you could get a better deal for yourself if you sent them everything set to go, but then your sales would likely suffer if your end-product didn't look good. Most of the work in music notation programs comes from making it look good - entering the notes is the quickest part of the job.