First, bear in mind that our shipping calculation feature depends on our shopping cart, so using Buy Now buttons, which bypass our cart and take the buyer directly to instant checkout for just one item at a time, would not be able to calculate shipping, although they can trigger collection of a shipping address during checkout.
It should not be possible for buyers to obtain free shipping by leaving the ZIP/Postal Code field blank in our cart, and the only orders with 0.00 Shipping that I am seeing in your E-junkie Transaction Log are orders for an ebook download. We only require entry of a postal code for countries that use postal codes, so if the buyer leaves that blank, clicking a checkout button would only prompt them to enter their postal code, so shipping charges can be calculated for their order before they would be able to proceed to checkout. PayPal and Google Checkout both also callback to our system to recalculate shipping based on the actual shipping address the buyer provides during checkout.
I have tested your cart button codes and been unable to reproduce any issue where free shipping could be obtained by leaving the postal code blank. If you could provide the URL of a page with cart buttons where you are seeing it's possible to take an order to checkout with free shipping, we can examine that particular case.