Pay Per Click
A funny thing happened not to long ago. Right after I complained to Wells Fargo that Ask Sponsored search had been billing my card without my permission, Ask took away the ability to do automatic billing for small budgets.
What had happened is that I had a small campaign set up, $10 a day for a client. It turns out they were only actually using less than $5 a day, but Ask was billing the card the full $10 a day and saving the money for future use.