Take a look at ShipWire.
They are a bit more expensive than Kunaki but are more flexible.
And they have warehouses both in the US and the UK.
See:
http://www.shipwire.com/help/global-warehous-network/Doing an API-to-API is fairly easy (very similar to Kunaki) and you can have your Infusionsoft integration in no time.
Best,
Bob
