Best plan for teenagers is a joint checking account (dedicated to the child's use) at your bank, with a debit card they can use for day-to-day spending. You can do allowance, etc with transfers, and most bankd will play ball as long as one person on the account (the parent) is 18.
What about those prepaid cards you can buy for cash at B&M stores? I'm pretty sure you can use those online.
Among the downsides of being in Canada is paying higher prices for anything that is not health care.
Groupies for the modern times we live in, if you wish.
One firm that's was trying to do kid payments was Virtual Piggy (now Oink): http://www.oink.com
I thought all the age-of-13 stuff, specifically, was because you can't legally enter yourself into a contract when you're younger than 13. Which is to say, you can't accept EULAs or Terms of Service (or you can, but they'll have no legal force.)
Some people even consider COPPA to be a form of censorship. It creates such a huge legal burden on websites if they allow < 13 year olds, that most websites have to ban them, thus limiting their right to speech.
http://www.lawchek.com/Library1/_books/contract/qanda/legala...
One interesting feature I wasn't aware of is that this does not generally apply to "necessities" or employment contracts.
But I don't think this one will stick around for very long. Prepaid cash/debit cards and automated deposits are an older and more proven way of doing this (not to mention just handing your kids $20 in cash every week). By giving them something that is accepted everywhere, you teach them how to handle limited amounts of money and how to resist temptations -- if I drop $5 on gas station snacks, I've just burned up a sizeable chunk of my allowance on something that will be gone in 10 minutes. Giving an allowance is great to teach young teenagers how to handle money before they get their own job, but I don't see Amazon breaking into this and locking kids into buying only from Amazon.