My wife and I will be in London for 6 days. I'm wondering how much we should prepay on Oyster Cards when we arrive. I know unused balances are refundable, but should we prepay 20, 40, 50 pounds? I don't want to be short as we travel around. Also, should be order online (we leave in 3 weeks from the U.S.) or wait until we arrive - is there a better price to purchase ahead of time? Thanks for any advice and recommendations.