A polite reminding, pertinent at this time of year for those more familiar with London's Congestion Charges and the ubiquitous  car parking charges, that London Buses operate with a "cash free" regime. Payment can be using Oyster Cards or using a "contactless" credit/debit card such as a validated Barclaycard. Whichever means of payment the same fares are charged. Note that whilst "pay as you go" Oyster Cards are non-personalised only one fare may be valid at anytime 

Be aware that for those frequently travelling, travellers may encounter the benefit of a "weekly cap"

For those "Seniors" visiting from "out of town"  their local freedom pass is only valid after 09:30 Monday to Friday (valid all day Saturday/Sunday) BUT IS NOT VALID on London Underground services