Have you looked at Braintree Payments?
Maybe Fee Fighters can find something for you.
If you don't have a merchant account or are OK with dumping your current one, you can go with Stripe. They integrate the gateway and the merchant account into one and essentially just expose it with an API. I use it for a product and it is really good. Their pricing is pretty good,(no flat rate, 2.9% of volume + $0.30 per transaction if I remember correctly).I don't know how they handle large transfers, however.