Thank you.
There is also another error, which has the same root. There are several places in the code, which must work the same direction and it seems there was a mix before. The orders store selectedCustomfield:value, the plugins stored selectedCustomfield:selectedCustomfield, from my point it would be logical to have custom:selectedCustomfield:value. But I didnt write that part. Anyway we must atm use what is there. For the version 2.2 we may redesign this part of the core. We use now everywhere selectedCustomfield:value or for plugins selectedCustomfield:custom. Should be backward compatible, but better for writing new plugins.
When you wanna help me in this area further, I added tags to the code "TODO productCartId" for easier finding the different places and understanding of the complex. Maybe I missed another occurence in the code, which is important. thanks for your help
Max