Monetise RSSMailer with pro features?

I launched a month ago with RSSMailer, I'm now at 724 signed up users, some have churned of course, but I'm still sending about 350 emails on an average day. (Not everyone gets an email every day)

Right now no one is paying for the product, everyone can use all the features for free. I'm thinking about ways I can monetise this. I'm leaning towards pro features.

One thing a lot of people are asking for is to pick a custom time for their email, right now everyone gets their email around 7:00 UTC. I thought that would be a nice first feature to add to the "pro pack".

Here are all my pro feature ideas:
  • Custom time for email
  • Custom days
  • Limit the number of feeds you can add for free
  • Add keywords to filter new items on
  • Option to send separate mail for some feed
  • API access
  • More customisation options (custom css?)
For the price I was thinking about 12$ per year.

What are your thoughts? Is adding pro features a good idea, and what do you think about the price?