What could ever cost extra on top of my plan?

Only extras you choose, never anything triggered automatically by your sales. You can add them when you sign up or later, on request.

• Before your number: priority registration (€199 one-time: your registration questionnaire goes to the tax office within one working day of your documents being complete; the office's own processing time is not covered); and the setup review (€249 one-time: a written statement on your VAT setup, covering sales channels, stock locations, OSS, EORI, the filing extension and points to watch, usually within 7 days of payment and complete information, once per client).
• Every year: the filing extension (Dauerfristverlängerung: €199 per calendar year if you file monthly, included at no extra charge if you file quarterly; booked in your account once your filing frequency is known, not at signup; we apply at the next window and the tax office decides; any special prepayment (Sondervorauszahlung) it asks for is separate from our fee and paid by you directly; a change of filing frequency applies from the following year); the stock module for stock moved between your warehouses in different EU countries (€499 a year, and €499 for each earlier year we file for you: we value the movements from your price list, report them on your German return and prepare the pro-forma invoices; it is part of your order when you tell us at signup that your goods are stored in more than one EU country); corrections of an already-filed return (Berichtigung, €99 each).
• Advisor time: a written response to a tax-office letter or one-off tax advice, €200 per hour, quoted before any work.
• Intrastat (the EU trade statistics return): only once the goods you move to or from other EU countries pass the German threshold (€1 million sent out or €3 million brought in a year); we prepare and file it on request, from the figures and documents you give us, billed by time and quoted before we start.
• Past returns: when you move to us with a German tax number already issued, returns for periods before you joined (€99 per past return) and an earlier year's annual return (€599).

After signup, in your account: OSS for cross-border sales to consumers in other EU countries (€300 to register, then €200 per quarter) and a German EORI number for customs (€199 one-time), which applies only where German customs is responsible for issuing you one, meaning your company has its own staff and premises in Germany. Most sellers need no EORI of their own: an EU company already holds one from its own country that is valid across the EU, and a company outside the EU normally has its freight forwarder declare as indirect representative on the forwarder's number. None of these is required for a routine German VAT setup.