Let's stop beating about the bush. How do you think the gigs that do 50 - 100 articles orders a day get done? Yes, they get outsourced for half price to either the Philippines or India (where English is widely enough taught), and the gig owner keeps the balance. So if you ever wanted to make a serious business out of this, you would have to do the same. Writing 10, 20 , 30 articles a day on your own is not going to happen, and not worth it. Until you can verify that demand is going to stay however, you'll just need to bite the bullet and do them all by hand. If you are ranking nicely in Fiverr for "article writing" and related keywords, then maybe demand will stay, in which case you have a choice to make regarding setting up an outsourcing operation.
Regarding Extras/Upsells for article writing.... easy. Say you will "SEO" the article to include such and such percentage of keywords used, appropriate use of headings, bold, italicized, etc. Easy extra right there for you.
If you wanted, you could proof read the articles before they went out to your clients (something a few sellers neglect to do and end up sending rubbish and getting bad reviews for it)
Good luck.