Fred,
You will have to have a merchant account setup in your app to be able to use the credit card fields on the cart, order forms and recurring billing. There is no way to hide the credit card fields in the cart as a feature of infusionsoft as of now.

But we have developed a script that will hide the credit card fields. That way your customers ONLY see the Paypal option to pay with.

If you would like more info let me know.
http://infusionpros.com/ip/services

attached is a screen shot of an example.

All my best,
Steve