New answers tagged letter-case
I find this to be helpful. http://msdn.microsoft.com/en-us/library/bb158706.aspx What's important is to remain consistent.
I don't have any "official" or user tested information about this, but in the examples you have detailed in your question, the sentence case works well for answer which are based around a response you would generally hear someone speak. It makes sense that "value" answers would be capitalized. i.e. Express Delivery Economy Delivery
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