Ask HN: Best payment solution for intermediary company

basically we are company that link between service provider and consumer, we take deposit (15% of full amount) from consumer (paid by credit card) and if the deal is done, consumer pay the remaining fund and we take a spread ultimately pay the rest to the service provider. so there is a lot of transactions, what is the best (cost-effective) solution available to startup? or is there a smarter work around?

Note - in the worst case scenario, about half of the deals cannot be done - i.e. we will have to refund the deposit to the customer

thank you very much in advanced!