Basic proofreading is available at $5 per 500 word page, to check for typos, spelling and grammar issues.
A more detailed critique is also available, to help with style, flow and other writing issues. Prices vary, please contact me for an estimate.
Even if the publisher to whom you are submitting work has an in-house editor, it is more professional to have your work edited by someone else, to have it as near perfect as possible.
It is almost impossible for a writer to catch all of the typos and errors in their own work, as our brains tend to see what we think we typed, instead of what was really typed. Despite spelling and grammar checks in word processing programs, these are not 100% accurate, either.