Another alternative is to buy a prepaid debit card and use it only. They will not be able to charge above whatever you paid for it to reflect. I use a separate account debit card for all of our online purchases and keep just enough in there to pay for those charges.

And, I check our online bank balances every day. When double charges do show up, they have always been mistakes and caught by our bank's fraud dept. They get really serious about it, too. I've had them flag purchases I've made because they raised a flag for whatever reason, then they've called me to make sure it was something I did myself.