PayPal last week launched a PayPal Invoicing API. Paypal says the new PayPal Invoicing API is available for integrations on ecommerce solution providers, marketplaces, and crowdsourcing platforms (among others) to allow users to create, send and process invoices all in one easy-to-use function.
According to PayPal, the Invoicing API enables merchants to invoice globally (invoices in 25 currencies and 200+ markets, with PayPal handling currency conversion), to send invoices at no cost (fee is on payment), to accept all major debit/credit cards, to manage payments, and to send reminders for unpaid invoices.
The documentation for the PayPal Invoicing API is available here.