Support
Email billing@suwebatu.co.uk with what happened, what you expected instead, and which invoice, client, or page it involves if relevant. We reply from that same address.
Check the status page first - if something's down platform-wide, it'll show there before we can reply individually.
Subscription, invoices for your own account, or payment issues - same address, mention it's a billing question.
See the docs for how invoices, quotes, credit notes, and approvals fit together.