OldCow,
I use GC (and Paypal) on my US site. Merchant id is set up on your Google Checkout login page (which is separate (or can be) from your analytics, Base, etc. accounts). The GC account information has to be mirrored on your zen-cart admin GC module page. You can't sign up for GC from your zen-cart admin, and zen-cart admin has to have all the correct info from your GC page. You actually can have multiple Google accounts (base, analytics, checkout, etc) and your analytic accounts may or may not be for the same websites as you GC account page. Setting up a GC account involves more steps and takes longer than setting up an analytics account because they need your tax id, bank account info, and must verify them. It makes sense for your Google Analytics account to monitor all your websites, but the way GC is set up, it is talking to only one website at a time (in fact it is talking to very specific secure pages on a website). In any case, since you want to keep the business accounting separate I would definitely set up a separate GC accounts for each business, even if there was a workaround to have one GC account for multiple websites. GC module for zen-cart is beta, GC on Google is beta. As you can see from this thread, you have to get EVERYTHING right in both your GC Account Page and ZC Admin page GC module to succeed in full Level II integration (that is, getting Google to login in to your website and record the buyer and decrement your inventory). The most frequent obstacles to Level II integration is that the zen-cart user did not buy a SSL certificate for their domain, did not specify SSL paths correctly (API call back page, will sometimes work for shared SSLs), or did not correctly set one of the many options, features, or otherwise follow the long and detailed directions, which admittedly can be quite challenging for even intermediate level computer users.
These are my opinions, but I am no expert in these matters. Hope this helps.
