There are tens if not dozens of posts here on FT alone about poor customer service experiences with Sixt which are only rectified after being posted publicly and Sixt then asking for a PM with the details.

This suggests an organization which does not react to complaints until they become an embarassment and then deals with the tiny % who do make a public fuss.

While it is nice that Sixt can be responsive through social media, it is the wrong way to go. A better move here is to take your business elsewhere and let Sixt fix itself.