Pay Now replaces the download icon when the latest invoice for that subscription or charge is not paid - typically because the card on file was declined. Clicking it opens Stripe's secure hosted invoice page in a new tab, where you can pay with any card. After paying, add or update the card under Default card on file so future renewals succeed. In the current version the Billing page itself doesn't say what happens if a subscription stays unpaid for a long time - that side is handled by Sprwt's central billing service - so contact support if you're unsure of your account's status.