Some of my work is in the Unix System V code… yeah, *nix runs through my blood.
“Hooks” refers to the ability to integrate your code into somebody else’s code. WordPress allows eStore and other plugins to hook into it (WordPress). eStore has the ability to hook into other “selected” plugins, and allows other “selected” plugins to hook into it (eStore). That’s what it means when you see Amin and Ivy talk about eStore integration.
The only way you are going to hook into eStore is to get down and dirty with the code, unless Amin reveals the presence of an eStore API.
On the subject of Google Checkout, please see:
https://support.tipsandtricks-hq.com/forums/topic/google-checkout-revisted#post-7904