After choosing to discard for buy, you can not discard for action, even if you have not yet discarded for action. Not sure if Bug or "feature", but should be changed either way.
This is not a bug. It's because you have to follow the instructions on Hamlet in order: First you get to discard for an action, then you get to discard for a buy.
It's just a terrible bit of interface design though - and didn't used to be this way - I believe its shortcomings are known by the team here...
The problem was in the previous interface you had to decide beforehand if you wanted discard for Action, Buy or both, and some reaction cards existed that might change your choice in between. Now the problem is that the interface doesn't make the order of those effects clear.