Hm, if you enable Shipping for the product in E-junkie Seller Admin, that triggers collection of a physical address during checkout but also tells Google Checkout that the order contains tangible goods, which apparently triggers their 15 min. delay to give the buyer a chance to cancel...
You could try this: Go into E-junkie Seller Admin > Payment Preferences and enable the "Only Authorize payments..." setting, which also disables the "Wait for pending payments..." setting there. I am not certain how this interacts with Google's 15 min. delay (we haven't actually tested your particular scenario to find out yet), but if GC sends us notice of an authorized (but not-yet-charged) payment as soon as the buyer finishes checkout and before GC starts the delay timer, then this could work for you. This would of course require you to manually approve charging each payment in your Google Checkout account, and I presume the option to do so would not be available until the 15 mins. had elapsed.