Currently, PayPal can be used to refill your account balance (automatic PayPal invoicing will be available in the near future).
For the time being, you will need to Refill your account credit which will automatically be applied to your outstanding invoice via PayPal. You can do this via your SkySilk Account Settings under My Account > Billing > Refill Credits. You will then be directed to PayPal where you can confirm the transaction. Once complete, the credit will automatically be applied to your invoice(s) and your account status will be marked as paid.
If you have any further questions, please feel free to Contact Us.