Ben is at the post office. He will/is going to post a letter.

English Explanation:

Since Ben is already at the post office, the action of posting a letter is either in progress or about to happen as a direct consequence of his location. 'He is going to post a letter' suggests a plan or intention that is about to be realized. 'He will post a letter' could be used to state a fact or a decision made now about his immediate future actions. Both are acceptable, but 'is going to' often fits better when the action is imminent due to current circumstances.