Paypal's Express Checkout system is down. It crashed shortly after a new update was attempted to be implemented to the Shopify/Paypal integration. We are waiting while Shopify & Paypal work on resolving both of these issues, and apologize for any inconvenience. It could be a week before Paypal Express Checkout is fully operational again.
If you would like to make a purchase in the meantime, an invoice can be sent directly to your Paypal email address. Just email Katie at firstname.lastname@example.org