Thanks for your detailed response.
Based on your response, it sounds like it is not possible to avoid passing on the tax_cart parameter. Thus, it is not possible for me to combine E-Junkie with PayPal's global tax settings, even if I leave all tax info blank in E-Junkie, correct?
PS: The reason I want to do this is similar to this thread: http://www.e-junkie.com/bb/topic/4442 (Australian finds sales tax in cart verbose and complex, bad UI)
The way I have it set up with E-Junkie WORKS, but I am dissatisfied with the user interface. I find it clunky and overly complex to make the customer choose their country from a giant list, as well as type their postal code before they commit to buy. (And, yes, I have set the default country to my country of choice.)
To get more specific, my main objection is that if you are from the default country and you simply press BUY you get a prompt that forces you to enter your postal code. But I don't even need to know their postal code, country is enough for my needs! So I don't want to force them to add their postal code, and I certainly don't want to prevent them from buying in order to make them fill in their darn postal code first!
My main concern is cart abandonment. I want the least number of barriers between customers and the buy button. :)
I am a happy customer, but am just trying to see how far I can take E-Junkie and make it work best for me and my clients.