Simplify the following text for a Grade 9 native English-speaking person.
Original: In accordance with his pre-election pledge , bus fares were frozen for four years , but then the cash fares on buses more than doubled while Oyster ( see below ) fares stayed the same .
Simplified: In accordance with his pre-election pledge , bus fares were frozen for four years , but then the standard single cash fare on buses more than doubled .