We can only support ClickBank with Buy Now buttons which bypass our cart and take buyers directly to instant checkout for just one item at a time. ClickBank's checkout system has proven incompatible with our more full-featured shopping cart, so Buy Nows were the best we could manage to support for ClickBank.
Presumably you were interested in ClickBank primarily for their affiliate system, so in that case you should use ClickBank as your sole payment processor for E-junkie, since ClickBank affiliates cannot earn a commission if the buyer uses any non-ClickBank checkout method.
You might consider using our built-in affiliate system instead, which can of course support our cart using checkouts via PayPal, Google Checkout or Authorize.Net:
http://www.e-junkie.com/ej/help.affiliates.htm