I must admit that I am quite happy with how this turned out... but it was more work that I first expected.
I tried to keep it simple but getting things simple is often more work behind the scenes:
displaying the correct currency when the user lands on the page for the first time (from the options of $, €, £) required geolocation
but geolocation is not always perfect so a selector to choose another currency
a cookie to store the currency so it doesn't need to be looked up every visit
options for quarterly and yearly billing
getting all this to look simple and work with good UX
Here it is: https://twitmate.com/twitter-management-plans.php