I have invested another day. It's now possible to add fixed a fixed amount/weight. Also the hardcoded transit email can be fixed that way.
Still my code is not totally finished:
There needs to be validation check in the new updateShipping function.
RMA seems to use "submitShippingNumber" as well. That's maybe not problematic. I don't use RMA at all. So no idea what this should do.
After a click on edit, the form looks like this:
Btw: For me this project is quite evident. My goal is clearly to push this into the core (after careful validations with merchants). If this doesn't workout, I won't supply anything in the future. Cause developping things for core, which doesn't get applied, is too costly:
I invest a lot of time to be consistent with the core, if it's only for my own store, all is much faster and simpler.
After a modification get declined, I have to rewrite again, since I need to use the code in an internal module instead of core.
If this important (and much wanted) feature can't be implemented, I lose faith, that there will come any real progess on other topics (like asm, attributes on bundles/virtualProducts and so on...).