Fixed odds betting

Just had a look at this for the first time on todays racing. A lot of the odds are very poor but I did find 4.33 on one horse that was being layed at 4.1 on the exchange and they took £200 with no problem which I was quite impressed by. Does anyone know the following though:

Do I pay commision on winnings?
If I lay it back on the exchange I assume they are treated as too different bets, so if I backed at 4.0 FO and layed at 4.0 exchange I would pay commision on the exchange bet?