I've used amazon payments, google checkout, and paypal.

Paypal
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Note, I refuse to use the euphemism "freeze your account" because that is not what paypal does. Paypal steals people's money.

PayPal will steal your money for all kinds of reasons, including the people who buy from you. You don't even have to be doing anything "weird" (unless selling people software online is weird.) When paypal stole our money, one of their demands to get it back was that we provide an invoice from the manufacturer of our software proving we had enough units to fill customer demand. This invoice had to be paper based, and we needed to scan it. It needed to be signed in blue ink by someone at the manufacturer. I kid you not, those were paypals demands.

We're selling software that we created, we're the manufacturer, and it is downloaded so, it is "created" the moment someone buys it. There is no risk that we'll run out of inventory. We'd been doing good sales, we had very happy customers, and no complaints.

Our crime? One of our customers had paid someone else via paypal, and that other person turned out to be a scammer. Paypal, when they went after that scammer, looked at everyone who paid him, and then everyone else those people had paid and then stole the money in those accounts as well. This was clearly an automated process and apparently happened instantaneously. It was clear from trying to talk to the "support" at paypal. by the way- attempting to work with paypal to resolve these things means dealing with people about as bright as the local DMV and MUCH less interested in helping you. As far as they are concerned, because their software decided so, you're a scammer trying to rip them off. They have no interest in actually telling you anything about what's going on. They just make arbitrary demands in the hope that eventually you'll give up.

Paypal was completely irrational, and dishonest. They lied to us many times. We did things they demanded and they just ignored it and then made more demands, or demanded the same things again.

As far as I'm concerned, Paypal boosts earnings by stealing their customers money. The only thing unusual about our account at that point, compared to previous periods, was we had left more money in it, since we'd not been withdrawing it to our bank account for a couple months. So there was a nice sum of cash for them to steal.

Amazon Payments
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We used Amazon at a different point in time. One day, Amazon just deleted our account, completely! Again an automated fraud thing, but they recognized the error. They mostly made things right. There ware a couple kindle books that had been purchased that they promised us a coupon to let us buy again and never delivered, but that's not a problem. Amazon's customer support was really disorganized and vague in their communication- they would often give us instructions that started with "Log into your account" when the account itself had been deleted. By trial and error we were able to figure out what needed to be done to get mostly everything back to where it needed to be. So, amazon, not really well organized, but not out to steal your money.

Google Checkout
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Used them for quite awhile. No real problems. No hiccups.. well, I think there was a problem with one order but we were able to resolve it, between us and the customer. Fortunately didn't ever have to deal with google "support" (since in my experience with other google products there isn't likely to be much.) Google's policies were much better than paypal, and they ran a tight ship that ran very well.

We looked at 2CO as well, and in our research they seemed to be a good place. If I needed to do this again, that would be my first choice, though I'd keep google checkout as a possibility, the real issue being more which one looked like a merchant account and which one looked like a branded pay button to our customers.