there is a risk with Fastpay that you might make a mistake with the numbers, and pay the money into the wrong account.
There is no reliable way of getting it back, and the bank will not reimburse you. Even if you have entered "MW Reliable Roofing Ltd" as the payee name, and the recipient's account name is actually "Woodbine Offshore Inc."
The same thing happens if you pay the money into an account number given to you by a person pretending to be the person you meant to pay.
I usually pay a pound first, and wait for the trader to confirm they have received it.