Secure Credit Card Submission

I am working with a non-profit environmental organization trying to set up an on-line donation form. They already have a credit card account (in the office), so they would rather not have to open another one for doing on-line transactions. Ideallythey would like a secure system that gives them all of the donation information and credit card #, which they would then enter into their machine, then send out paper confirmation.
What would be the least costly way of handling these transactions securely?