How come the amount to send a message changes?
In Planet-Tolkien the weather plays a big part in many of the features you use and experience. This is also the case when sending Messages. In middle-earth there weren't really any weather hardy methods of transport that could offer the messengers protection, also messengers just like you and me are human (well not all but most are) and sometimes just don't want to go and deliver messages in the rain, hurricanes, hail storms or whatever else happens to be the weather for your home location on that day. Not unless they are being paid a little extra that is! It's just the way things work in Planet-Tolkien, sorry =)