WooCommerce Payments Processor
Setup is fairly easy:
Activate the WooCommerce Tools plugin.
Activate the payment option:
Found within the Sprout Invoices settings, on the Payment Settings tab.
Setup your WooCommerce store and its own payment gateways and you're done.
YOUR CART IS CURRENTLY EMPTY.: The invoice payment product isn't being added to the customer's cart.
Are you using WP Engine? At this time WooComerce Payment processor isn't yet compatible with the WPEngine because of their caching options interfering with non-logged in users.