You can set your website to only accept orders once they reach a certain pricing or value threshold.

For example, you can set your website up so that you only accept orders that are valued at $2 or more.  This will prevent too many customer from order just low value single cards.

To do this, go to you Admin -> Account and Payment section.

You just need to enter the minimum value of orders that you would like your customers to make before they can process a website order.