In order to use PayPal as a payment method from your site, Yahoo! needs to be able to communicate with PayPal on your behalf. This communication happens by using several PayPal APIs (application programming interface) which allow the two separate systems to communicate securely using an agreed upon format. Each set of actions either performed by a buyer (logging in to PayPal from a link on a merchant's site) or a merchant (capturing funds for an order) requires the use of PayPal APIs.
As part of the process of setting up PayPal, you must log in to PayPal and enable the PayPal APIs that are specific to the Yahoo! integration.
To enable API access with PayPal:
If you enable API access with PayPal and later change your email address with PayPal, this will cause a communication error between the systems. Customers using PayPal as a payment method may not be able to complete their order with this payment method. Merchants should not change their email address with PayPal once the PayPal APIs are enabled.If you've changed your email address with PayPal and are experiencing problems, please do the following: