When trying to edit Shipping by weight values all styling and formatting disappears and the update command does not work.
This is Critical as it means if a value needs to change the system will "crash".
The Delete command also does not work and causes the same effect.
Replication information:
Go to Shipping Rate Computation Methods
Configure Shipping By Weight
Add more than one entry ie
1. weight 0 - 250 and charge 1
2. weight 251 - 500 and charge 2
Then edit Entry nr 2
I have replicated this on your store demo site also.