You can top up online. Just open an account at the Transport for London web site. You can then add credit by paying with a credit or debit card.
You can even set it so that your card tops up automatically once your credit falls below a certain level. Unfortunately, you still need to go to a tube station, as far as I am aware, to activate it, which can be done by passing through the barrier.
Presumably you occasionally use the tube, even if your card is mainly for bus travel, so that should be feasible. It takes 24 hours for credit added online to register on the card, so if you add credit today you will be able to 'collect' it tomorrow.
Visitor Oyster card is one of the cheapest ways to pay for single journeys on London's transport network. Learn how to use your card and top up as you go. Buy the London Visitor Oyster Card, valid for all of the London Underground, buses, trams & even some National Rail services in 5 different pre-paid smartcard options. Validities are purchased as prices in £(GBP) and include the price of the card plus the £5.00 activation fee. Visitor Oyster Card is not valid on. Nov 18, 2016. NOTE: The Visitor Oyster card activation fee changed from £3 to £5 beginning September 17, 2017. The standard Oyster card allows visitors to add travelcards and bus and tram passes onto the card and the Visitor Oyster card does not. This is unlikely to matter for those staying for only a short.