You might be surprised how often a deep graph of microservices ends up rereading the same prior transactions over the course of stateful payment processing and on-demand payouts. DynamoDB can give you 2.6 million short reads of base load (1 RCU/s provisioned) for $0.12 per month, which would make a $65 alternative (2.6 * $5 + 2.6 * $20) a hard sell.