Hi,
I have great experience with Amazon MWS API. Once I have written script for my client which need to sync data with multiple countires amazon store. Amazon USA API is more mature than others. If it is working with UK, it should be ok with USA. As you said that it alsways returned you that fast delivery is not available then it can be a potential bug.
Send me that script, I will fix the issue. I have very good profile on another freelancer website, if you be having any doubt on my skills, I can send you that link or show you my work on skype/teamviewer sharing.
Let me know if you will be interested.
Kalim