What exactly is included in the €99/month?

That is the Standard plan for a business that already holds a German tax number: every monthly or quarterly VAT return filed with the German tax office (Finanzamt), the annual VAT return, the EU sales list (Zusammenfassende Meldung), an automatic double check of every sales report and return before filing, every tax-office letter read and explained in plain English with the deadline named, your accountant in copy and a dashboard in English. Extras such as the filing extension, the stock module or corrections are bought when you need them. On monthly billing the annual return for a year is included when we file at least July to December of that year; join later and that first year's annual return is a one-time €599. Not included: incoming-invoice review, tax advisory and legal advice, which are advisory on request at €200/hr. The EU One-Stop-Shop (OSS) is a separate add-on, not part of the €99/month.