Sorry.. typing faster than I was reading. The URI::query command should return whatever is between mailDrop= and the next & or the end of the string, regardless of the length of that value. Are you wanting to only use the first four characters of the mailDrop parameter value even if it's longer in some cases? Do you see a & in the redirect string (or what exactly do you see in the string and is that what the mailDrop parameter value is set to in the original URI)?
If you do want to take only the first four characters of the mailDrop parameter value, you can use this:
HTTP::redirect "http://newsite.com/buy.aspx?Quantity=0&CouponCode=[string range [URI::query "?&[HTTP::query]" &mailDrop] 0 3]"
Aaron