Customers checking out with PayPal Website Payments Standard (formerly referred to as “PayPal IPN”) will complete the order in the store and once coming to the choice of the payment method, they can select PayPal . There will be a large PayPal button for them to click, they are directed to a page that allows them to log into their PayPal account or to pay by credit card without having to sign up for a PayPal account. After the payment completion, customers are returned to the store.
Website Payments Standard is the fastest way to set up online payment on a website to accept credit cards, debit cards, bank transfers, and PayPal. The integration is easy (HTML based). In the case of ZenCart, if there is any problem in PayPal’s ability to communicate to the store server, the order will not be passed to the store.
PayPal Express Checkout gives the customer two options: They can jump over to the PayPal site to login to their account before completing checkout on the store (which allows them to select their address information there and never have to re-type their address details on the store site, thus the “express” part of the transaction) and then choosing shipping choices and discounts/coupons etc before completing the order and confirm the payment.
The other possibility is to go to the PayPal site to login to their account after making shipping/payment/coupon selections on the store site (and creating an account on the store and typing their address info on the store), much like they do with PayPal Website Payments Standard.
One important benefit is that Express Checkout does not rely entirely on the IPN communications to the store in order to release the order. Instead, it stores the order immediately when payment is completed. It doesn’t have to rely on the PayPal server to talk to the store server in order to save the order. IPN updates are still useful in case of refunds or adjustments to the order and are reflected in the store’s order history.
The integration of PayPal Express Checkout is more complex and is based on API’s.