PayPal Invoicing makes it simple to send professional, customized invoices. PayPal emails your customer a link to the invoice or you can supply a link in your own email. Your customer then views invoice details and securely pays with a credit card, debit card, PayPal, or PayPal Credit. Whichever payment method the customer chooses, you generally receive your money in minutes.

You can also manage your invoices. You can view the status of invoices, manually record payments, and more. You can create and manage your invoices from the PayPal Website, PayPal Business mobile app, or our Invoicing API.

Sending and managing invoices costs you nothing. You pay PayPal’s standard processing fee only when you get paid online.

You can use the Invoicing API to integrate invoicing directly into your order management system, or you can call the Invoicing API on behalf of another merchant after obtaining third-party permissions.

See also:
About PayPal Invoicing    
PayPal Invoicing API Documentation