Having used both Google check and Paypal (and now you have express checkout and direct payment), clearly, Paypal is not wanting to cede the market to Google.
Google checkout has the advantage of 0% fees the rest of the year! Not 3%, 0%. Next year, you get a percentage of the transaction fees in adwords credits. As much as spend on Adwords, this is better for us than free processing even. So, we definitely supported Google Checkout. Definitely, the google code has a lot of bugs, and, unless you are a programmer, not easy to find fixes for all of them. One of the biggest issues is shipping quotes if you use real time quote modules, but, your mileage may vary.
Paypal of course would be the more traditional route once direct payments are supported with stock Zen Cart. As it will appear as any other merchant account always did. And, since you can do real credit card payments and traditional paypal, far easier to meet the 10k monthly for a 2.2% rate. And, I don't see where there are ANY additional fees for business cards, etc.
Some won't use Google Checkout because it's Google, some will. So, for me, the best answer is to use both!
The potential loser in all this is the traditional merchant gateways and providers.
Yes, those business card fees and others really add up.
Yes, thanks for the reply back on the instant money available. Yes, any given transaction may be held up and that is fine. But if you have a lot of them in any given day, the odds are certain all of them would not be. So, it's far ahead of waiting 2-5 days! This is a great side benefit. And, one leg up on Google checkout.



