I found Hetzner's domain robot to be easy to use and cheap: https://www.hetzner.com/registrationrobot

The major downside is the limited range of TLD's it allows you to register: .de, .com, .net, .org, .info, .biz, .eu and .at (other domains are available through Hetzner's normal webshop)

With domain robot have full control over handles, name server entries, auth codes, etc. Annual fees are low (from EUR 3.28 to EUR 12.90) and don't increase year over year. The only additional cost are if you use their name servers (0.50 Euro/year).

Automation is only available through email interface.