Hi Angela,
Yes, eCrater is a FREE marketplace and online store. When you register with eCRATER you receive a customizable free online store. In addition, all your products get listed in their free online marketplace. Hidden costs? Not that I know of. If you've got more questions, I suggest you try and visit their community (forum). I'm pretty sure you'll find the answers you're looking and probably even more.
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I have not personally used Google Checkout but here are a few links that might help you out:
Fees:
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User testimonials:
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We also have quite a number of members using Google checkout, I hope they can share their experiences with you.
Cheers! :)