Amazon is a wonderful company. I used Amazon prime and they canceled the subscription for me after the trial was up even though I was supposed to do it myself (I forgot)! You have nothing to worry about. Take advantage of the free expedited shipping.
That's odd, regarding the validation charge. I don't remember getting such a charge when I signed up, but that was years ago. But Half.com has nothing to do with Amazon, right? I don't know anything about Ebay as I've never used it. Not sure what the charge is about though.
I don't think you get the validation money back. Paypal does much the same when checking a bank account but they do it in reverse and give you a few cents. I've had amazon prime for the past year. I order everything I can from amazon. If I had the money I'd just get my groceries off there and have them delivered to my door so it's worth it to me to be able to select 2 day shipping whenever I feel like it without worrying about extra costs. I've never had any problems dealing with amazon and I doubt you would have any problems canceling if you don't want to keep it.