Following the theme of my last LockerGnome article on saving money with tech, How to Save Money with the Best Light Bulb, I continue that theme with this article on how to save money and protect yourself with online purchases using virtual credit card numbers.

Virtual Credit Card Numbers
This is a feature of many credit cards that hardly anyone knows about. I’ve been using them for seven or eight years. They are fantastic! “Virtual accounts” is a free service available on many credit card Web sites. Some may even have an app you can install on your desktop. I don’t know of any mobile apps that support this, but please, if you’re aware of one, let us know in the comments — that would be a game changer! If your credit card supports this feature, you can use either its Web interface or its app, if it has one, to generate a new credit card number on the fly at the moment you need it.

So, suppose you want to buy something from say, Amazon.com (or anywhere else, for that matter, even catalog orders on an 800 number, or your utilities… any card payment by phone works, too). When it tells you the total, you generate a new credit card number, and here’s the cool part: you enter a maximum amount that can ever be charged to that number and you can also enter an expiration date as short as two months from now or as long as your real card’s expiration date.

This comes in really handy when your vendors attempt to charge more than the agreed upon amount. This happens more often than you think and from places you’d think were reputable. For example, when I purchased an upgrade to Quicken, I was charged 50 cents more than the agreed upon amount and the transaction was blocked. You may have to call the vendor to find out the total amount because sometimes they won’t give it to you until after you give them your credit card number. You actually need to know the total when you generate the virtual card number. As a workaround, you can generate a number for $1 (the minimum on most virtual cards), move through the order process (but don’t complete it) to get the total, then you can increase the amount on the virtual card before you submit… as soon as you find out the real total.

Once the vendor charges your card and the max value has been consumed on that virtual card number, that number will no longer accept any more charges. You can, however, increase the amount if you need to. In any case, you don’t have to worry about hackers stealing that vendor’s database and stealing your card number. As an example, here’s a virtual card number I just created:

Virtual Credit Card
Real Life Virtual Credit Card Number

Yep! That’s a real credit card number! But only $1 can be charged to it, and I also just “closed” that number, so not even $1 can be charged to it anymore. It’s now a dead number. It was a valid and active number for about five minutes.

How Do You Get Started with Virtual Credit Card Numbers?
First, you have to find out if your credit card supports it. This will not be easy, because most credit card employees have no idea their company offers such a feature. Feel free to call first and ask. You might get lucky and the employee that answers your call actually knows, but don’t give up even if they say “No, we don’t.” I wish I could give you a list of cards that do support this, but I can only guarantee for certain that Citi MasterCards have this feature. If you have a Citi Card, go to citicards.com, log in, open the “Tools & Services” menu, and choose “Get a Virtual Account Number” and follow the directions there.

I believe the PayPal Visa card also supports this. If anyone knows of any others, please share it with us in the comments below.

You can also search the Web for credit cards that support virtual credit cards. I highly recommend starting with groups.google.com, the best kept secret of the Internet. If you can’t find the exact answer, at least you’ll find posts in groups related to it, then you can join that group and ask your question and usually get an answer within hours (depending on how active that group is).

Here’s a run down of the benefits of using a virtual credit card number:

No one can overcharge you.

It doesn’t matter if hackers get a hold of your vendor’s database with your credit card number in it. Once your vendor charges the full amount to it, the card number is useless.

You can disable any virtual card number at any time.

You can prevent services from doing automatic charges or “renewals” monthly or yearly, like Xbox Live and XNA Developers Club! When you’re done with a subscription, just disable the card. No need to bother with going through the mess of trying to convince a hard salesman in the cancelation department that yes, you really do want to unsubscribe. All too frequently, they’ll keep you on anyway.

Have a separate credit card number for each purchase.

Have a separate credit card number for each merchant.

Works with all online credit card purchases and all phone orders… anyplace that doesn’t require physical access to your credit card.

By the way, I’m not promoting Citi Cards. Actually, if you can find another card with this feature, you might be better off going with it. Citi tends to have higher interest rates.

Have you used this feature? Are you aware of other cards that support this? Let us know in the comments below.