Ripple's fees per transaction I have Toast Wallet in order to store my XRP properly. Now I want to perform my first transaction from my wallet to my exchange in order to understand how it works.

In a note of the Toast Wallet is written: 
"The Ripple Network requires a minimum 20 XRP balance at all times and charges fees based on congestion even for failed transactions"

Do you know how much are the charges fees more or less and if even in my case I have to pay that?

With what probability I receive a failed transaction? (supposing I'm typing all address and informations required well) .. [EG If there is a lot of traffic in the network is possible that I receive a refund and so I lost my fees?   

How many XRPs can I transfer? Can I transfer only 1 XRP? (obviously I must have 20 XRPs on my balance wallet)

Thanks for the ansewrs,
Marco.

P.S. Sorry for my bad english but I'm Italian :)
The current minimum transaction cost required by the network for a standard transaction is 0.00001 XRP (10 drops). It sometimes increases due to higher than useal load.