Another option here is to check out Intuit Billing Manager. It's fast, easy, and does a great job at invoicing. www.billingmanager.com.
(Full disclosure: I work for Intuit)
Hello Richard Morochove
Thanks for the excellent review.
We are working on supporting other payment gateways in Zoho Invoice. We will be announcing support for Google Checkout this week, more to follow.
Regards
Siva
Team Zoho