Since v1.3.7, Zen Cart has built-in integration for PayPal Express Checkout.
This allows your customers the convenience of shortening the checkout process by logging in to their PayPal account, selecting a shipping address, and returning to your site to confirm their order.
To use Express Checkout, you only need a PayPal "business" account. You do *not* need to subscribe to "Website Payments Pro" services.

How it Works

Once the customer (logged in or not) has something in their cart, the Express Checkout button will show on the Shopping Cart page just below the regular Checkout button. Clicking that button begins the Express Checkout process, taking them to PayPal to choose their funding source and shipping address. Then they are returned to your site to complete checkout.

It bypasses your normal Shipping page by auto-selecting the least-expensive shipping method available for their shipping address, as configured by your store's shipping options.
You can force it to allow the customer to go directly to the shipping page by setting the appropriate switch in the PayPal Express Checkout payment module in the Zen Cart admin.
The customer can change their shipping choice by clicking the Edit button next to their shipping address on the confirmation page.

Payment Page
It bypasses the payment page because it knows that PayPal is their choice at this point in the transaction.
HOWEVER, if your store has coupons activated or has gift certificate support enabled, then the customer will see the Payment page before they get to the Confirmation page -- so that they can enter their coupon code or select the amount of gift certificates they wish to apply to their order.

Confirmation Page
If the customer has already gone to the PayPal site and selected their funding source and shipping address, then when they click "confirm" on your Confirmation page, their order will be processed, payment collected, and their order number will be displayed on the Checkout-Success page. Their order will be immediately available in the Admin area, and in most cases "released" immediately, making downloads etc available right away.

If the customer goes back and does additional shopping, or if they proceed through checkout in the traditional fashion, they may still choose PayPal on the Payments page, or choose from the other enabled payment modules. In this case, they will be taken to the PayPal website to confirm their payment and shipping details after they Confirm their order on your site.

New Customers
If the customer visiting your store doesn't already have an account and uses PayPal Express Checkout, an account is created automatically for them, and their password is emailed to them in the Welcome Email which sends when they return from the PayPal site after entering their PayPal login details and selecting their funding/shipping options.

Customers can then use this account to log in again later and re-access their downloadable items or check on order status.

For those shopowners wishing to not create accounts for their customers, they may set the switch in the PayPal Express Checkout module which marks the account as "temporary" and skips the welcome email and sending the password to the customer. The account will still be created and remain available until the same customer does another Express Checkout purchase, at which time the temporary account will be deleted and a new one created.
NOTE: Accounts are made permanent and the welcome email+password are automatically sent regardless of this "temporary" setting *if* the customer's order contains downloadable items or gift certificates.

Website Payments Pro
Looking for Website Payments Pro? Click here for more information on the various PayPal options.