Oyster card refill

The Oyster ticket machines will vary as to whether they accept coins, bills, or credit cards, but they will ask for payment. Top-ups are also available at many "convenience stores" and kiosks, which will again vary in which forms of payment they accept. Unless your sister lived there, she probably did not activate the "Auto Top-Up" feature.

You don't have to use a credit card to top up. This summer we used cash to top up our Oyster cards. Just follow the prompts on the screen, choose the option for a cash top up, feed the machine the cash, and then hold the card up against the 'tap pad' that's on the front of the machine. It's really easy and quick.

Be careful to avoid mixing credit card payments and cash payments on your Oyster card. A few years ago when I visited London I used my credit card to pay for the Oyster card (5 pounds I believe) and to add money for transportation. During the course of my visit I topped off the card using my credit card but also made the mistake of topping it off once with cash. At the end of my vacation I went to a Tube station to get a refund of the card and the remaining fare. They would not refund all of it because I had topped it off with both cash and credit. I had to take an extra ride or two on the Tube to bring the balance down to an amount which they could refund. I wasted maybe 5 pounds in order to get a refund. So, the recommendation is to only use a credit card to buy the Oyster card and to top it off.

The above is good advivce for travellers. If you have used different modes for top up the normal refund mechanism is a cheque posted to your home address.

Why not just leave the card active with its balance for your next trip (which could be a friend or family member)? If it has 5 GBP, then you are ready to roll onto the Tube coming in to Heathrow even without currency, and no need to deal with Heathrow only having currency exchanges instead of real ATMs.

To followup Marco's comment, I read somewhere (maybe Rick Stevee book on London) that refund checks are mailed to addresses only located in the UK.