I think for you to advertise such service better, it would be nice for you to actually put up some link to some of your past work or provide a site where we can verify your claims as a good writer if not everyone can just come up and offer such service even with poor skills
Check the edited opening post. I'm going to cut to the chase here, if my writing skills meet your needs you'll see it as you review the article. If not, your not expected to pay.
While this seems to be a good tactic in case you do not have any portfolio but remember this open ended offer can be miss used by the employees. Some scammer may offer you to write and do not pay you even if he likes your article.
Why not until you get any offers try writing your own articles and use them as your portfolio, this way not only people will know how you can write but at the same time you do not have to come across an awkward situation where you write and not get paid.