What happens after a subscription service/certificate expires?

You can renew the service before it expires. After renewal, you may continue to use your existing certificates. If your signature quota is used up, you can purchase additional add-on packs separately; the validity period of the add-on pack is consistent with that of your primary plan. (In most cases, the two expire at almost the same time. Strictly speaking, the service is activated after the certificate is issued, so the certificate expiration time is earlier than the service expiration time.)