Shopify/active_merchant — pull request #657 by duff (2013-04-10 21:42:03+00:00)
Title: Paypal Express: Lessen parameter requirements

For reference transactions, Paypal Express does not require a payment
type, an invoice_id, a description, or an IP. We shouldn't require them
either.

They can certainly be specified and we pass them to Paypal
appropriately, but they're not required.