Thanks, Gary, but I don't do copy editing. That is, I'll point out any recurring mistakes in punctuation that I see, but I won't go through and fix them all. I'll fix a representative number, and then expect my client to get the point and do the rest. I work with plot, characterization, etc.
Anyone who wants a copy editor needs to find someone who has taken classes in that area.
But a writer doesn't have to learn every last rule of punctuation. A surprising number need to go back and learn the ones they supposedly learned in school, when they were about 12 or 13. Failing that, they simply need to get a feeling for what is right and what is wrong. Either will put them on a level with the typical agent or acquiring editor. If the book is accepted, then a copy editor will be hired to look out for all the obscure little rules the writer may have broken.