I got an email response from Amazon customer service:
Thank you for writing to us at Amazon.com.
I’m sorry for the trouble you had with your shopping cart.
I’ve reported the problem, and our technical team is working on taking care of it right now.
Often these errors are corrected after only a short time, so please try again after two or three days.
I understand that this might be causing you lot of inconvenience. Please understand that we are doing our best to resolve this problem, but technical glitches cannot be predicted and at times it is unmanageable.
Thanks for your patience while we fix this problem and thank you for shopping at Amazon.com.
Amazon.com Customer Service