Thanks for the more details - I understand now.
Yes, you are correct there is no current way to enable two levels of free shipping.
The only thing I can think of without having to do some type of coding would be to offer a coupon for the terms you wish on the registered mail. There is a coupon on the system for free shipping so you could modify it to meet your requirements.
you can add info to your pages using the language definitions or blocks for the pages that tell all of your customers the if they prefer Registered Mail and the order is over $50 use this coupon for free shipping.
Could even be an upselling advantage for them to spend more
Hope this helps you.
Lee