Uber/Seamless/Postmates/Airbnb or any other middleman service. There's no reason why they should be getting away with extracting 30-40% of the revenue for facilitating a simple transaction.
These companies are also taking a giant cut but still losing money. Granted they are spending money on things that an open source alternative wouldn't necessarily spend money on, but I don't think very many of them are anywhere close to being profitable even without the extra spending.