Many of the entities that process transactions do fraud monitoring. Oftentimes, the fraud monitoring has different levels of time spent depending on amount: und

Many of the entities that process transactions do fraud monitoring. Oftentimes, the fraud monitoring has different levels of time spent depending on amount: under $10 and the system just ignores it, under $1000 and a computer uses some heuristics, and above that, it might be flagged for human review. These aren't exactly 5% of the cost of the transaction, but they do show that the cost to process the transaction does increase as the amount increases, and fraud prevention, detection, and responsibility is how companies justify charging a percentage on a transaction.