Proofreading is the last stage pre-publication, catching those pesky errors that slipped through the net. That's all it is.
If you're sure your book is polished, I'll offer a low fee for proofreading. I do have to see the manuscript ahead, before accepting a proofreading order. This is to be sure it's truly the final stage before you go live with your work.
In some cases, I go back to an author and confirm their work is indeed ready for me to proofread, whereas I'll have to advise others to have their work professionally edited before this stage. Of course, I will offer an editing slot and a competitive rate at the same time.