I do lots of ghostwriting and I charge per hour but more professional writers seem to charge per post...I just find it easier for my billing practices to charge per hour. (I'm also doing the research to create the blog posts, etc. so that falls under a regular hourly rate as well...)
You've already quoted so give it a shot and if it isn't going to work out to your advantage you can always approach the next client with a new rate/perespective (congrats on the new client!

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