Any of PayPal's products will let your customers pay via credit card. That includes PayPal Express Checkout, PayPal Website Payments Standard, and Website Payments Pro.
There is a fee for the Website Payments Pro service.

All three PayPal options are built-in to Zen Cart. You can configure them like this: www.zen-cart.com/getpaypal

If you have questions about which PayPal product is best for you, it's best that you choose Express Checkout, and maybe also Website Payments Pro.
You can contact PayPal Merchant Services for more clarification on which is best for you.