It's possible that the seller misunderstood but to me the original question seemed very clear.   

I also think its a legitimate question, especially if the listing stated item would be sent via satchel but didn't specify a specific price.  There is a reasonable difference in price between 500gm and  3kg. 

As a seller I always answer questions with as much detail as necessary.  I wouldn't ask why unless I needed extra info to be helpful.  Like you OP, this seller would have put me off too!