What is the quickest way to offer your SaaS customers credit card billing?

I have a SaaS product and want to offer my customers the ability to accept credit cards themselves. What's the quickest way to set this up?

EDIT: I'm starting to think that giving the SaaS user a way to embed a PayPal button would be "quickest"/"easiest". I'm cutting the feature for the time being but I would love to see any other alternatives.

Take a look at the offerings from:

• cheddargetter.com
• chargify.com
• recurly.com

Typically these services provide ways for YOU to charge YOUR customers. But maybe they have options so that your customers can charge their customers.

I have issues with this type of service because many of them lock you into their choice of gateway providers, their APIs, and their price increases.

Recently Chargify completely did away with their free tier, causing all the small companies who had invested tons of time implementing the Chargify API to have to either start paying for the service, or scrap all the work they had done. Luckily I was still just evaluating them, so I wasn't burned. But they did go from top on my list to bottom overnight.

I just came across a new one, but don't know much about them (are they PCI compliant, are they stable, will they be around in a few years, etc.). On paper they look good, as they support numerous gateways and offer a migration tool from the others. See them at http://zingybill.com/

One thing to keep in mind is that you cannot act as an agent that processes payments for your customers without getting on the wrong side of the credit card companies.