I've been doing a side gig proofreading a company's technical documentation for the past couple of months. The workflow is the company sends be a batch of technical articles for me to proofread and change, I do it in my own time and send them back the fixed copies of the files. They review my changes and there is often a bit of back and forth discussion about the changes. The company pays me X cents per word in the original article after I finish proofreading it. The issue I'm wondering about is that during the back and forth discussion, the company will sometimes add a couple of sentences to the article and ask me to verify whether the new sentences make sense. Since these sentences were not part of the original article I technically don't get paid for that work.
I have two questions about this situation:
How can I ask the company to pay me for those sentences without coming off as a penny pincher? Should I be getting paid per word of the completed article instead? What's a clean way of bringing up such a topic without annoying the employer?
A couple of days ago I read on this site that as a freelancer I should be asking to be paid in advance of doing any work. Given that they have been paying me for my work, is this something that I should bring up?
I've looked around the website but haven't been able to find something that matches my situation closely enough, so I decided to ask my own question.