The way Kunaki works with Nanacast is it has to calculate and charge for shipping. The reason is because if you're shipping internationally, the shipping is significantly higher than US. Kunaki provides a nice API for this, so that's how it integrates.
If you want to use a flat shipping rate, then Kunaki probably isn't the best fulfillment house to use. Vervante would be better.
Keywords: Shipping, Kunaki Fulfillment