Gap-Free Invoice Numbering: §14 UStG & GoBD in WooCommerce

What does §14 UStG require?

Under §14 UStG, every invoice must carry a uniquely assigned, sequential number. This number serves to clearly identify the record and ensures that no invoice is in circulation twice. In practice this means: the number must make clear that it was used only once.

It is important to distinguish between "unique" and "gap-free": a gap in the number sequence (for example, because a started order was abandoned) is legally tolerable — provided it can be plausibly explained. But that is exactly what becomes a burden in a dispute with the tax office. Sequent therefore goes one step further and guarantees gap-free numbering, so there is never anything to explain.

The technical challenge: concurrent orders

Picture two customers completing their order at the very same moment. Both processes read the current counter value, increment it by one, and write it back. Without protection, both read the same starting value — the result is a duplicate number or a skipped position. In a shop with parallel traffic this is not an edge case but everyday reality.

The solution: a database lock

Sequent assigns the invoice number via a serialized counter inside the same database transaction that creates the invoice record. A database lock (a lock on the counter) ensures that, at any given moment, only one process can draw the next number. So the sequence stays strictly sequential and gap-free even under concurrent checkouts.

Crucially: a number is only consumed once the invoice record is actually committed. If the process is aborted, the transaction is rolled back — and no number is consumed. So no gap arises even when an order fails.

1. Step 1 – Acquire the lock. When creating the invoice, Sequent opens a database transaction and locks the counter for that document series. Other parallel processes wait until the lock is released.
2. Step 2 – Draw the number. Inside the lock, the counter is incremented by one and the next sequential number is reserved — guaranteed to be assigned only once.
3. Step 3 – Commit the record. The invoice and the new counter value are stored together in the same transaction. Only with the commit is the number finally consumed.
4. Step 4 – Rollback without a gap. If saving fails, everything is rolled back: the counter returns to its previous value, the number is not consumed — no gap.

Immutability: the record as a snapshot

A core GoBD principle is the immutability of records. As soon as an invoice is issued, Sequent stores it as an unchangeable snapshot. The record can no longer be edited afterwards — neither the amount, the date, nor the recipient.

If an invoice has to be corrected, it is done exclusively via a cancellation or credit note — never by changing the original afterwards. These correction documents also receive their own sequential number from the same locked series. This keeps the entire document chain gap-free and traceable.

§14 UStG	Requires a unique, sequential invoice number
Gaps	Tolerable if explainable — Sequent guarantees gap-free anyway
Assignment	Serialized counter in the same DB transaction, via a database lock
Rollback	An aborted process consumes no number — no gap
Immutability	Issued invoice stored as an unchangeable snapshot (GoBD)
Corrections	Exclusively via cancellation / credit notes

Small business, reverse charge & retention

Sequent automatically accounts for the most important VAT special cases:

§19 UStG – small-business scheme

If your turnover is below the thresholds of €25,000 (previous year) and €100,000 (current year), you can use the small-business scheme. Sequent then automatically adds the §19 UStG note and shows no VAT.

§13b UStG – reverse charge

For the reverse charge (tax liability of the recipient, reverse charge under §13b UStG) — for example with certain intra-community B2B supplies — Sequent adds the corresponding reverse-charge note and issues the invoice without German VAT.

GoBD – retention periods

The GoBD set clear retention periods: 8 years for invoices and 10 years for books and accounting records. Sequent files every record in an audit-proof, immutable way, so the periods can be met without difficulty.

§19 UStG	Small business: €25,000 (previous year) / €100,000 (current year); §19 UStG note, no VAT
§13b UStG	Reverse charge — tax liability of the recipient; automatic note
Invoice retention	8 years (GoBD)
Books retention	10 years (GoBD)
Credit notes	Cancellation / credit notes fully supported

How Sequent implements it in WooCommerce

• Gap-free, sequential numbers via a counter serialized by a database lock — even under concurrent orders.
• Rollback-safe: a number is only consumed when the record is committed — no aborted checkout creates a gap.
• Immutable records as snapshots; corrections exclusively via cancellation / credit notes.
• Automatic §19 and §13b notes plus correct VAT handling.
• Entirely inside your WordPress installation — no external cloud, no data sharing.

Frequently asked questions

Are gaps in invoice numbering forbidden?

§14 UStG requires a unique, sequential invoice number. Gaps are legally tolerable as long as they can be plausibly explained. Sequent, however, guarantees gap-free numbering, so there is never anything to explain.

How does Sequent prevent duplicate numbers under concurrent orders?

Sequent assigns the invoice number via a serialized counter inside the same database transaction that creates the record. A database lock ensures that, even under concurrent checkouts, every number is strictly sequential and assigned exactly once. A number is only consumed once the record actually commits.

Can an issued invoice be changed afterwards?

No. Under the GoBD, immutability is a core principle. An issued invoice is stored as an unchangeable snapshot. Corrections are made via cancellation or credit notes, never by editing the original.

How long must invoices be kept?

Under the GoBD, 8 years apply for invoices and 10 years for books and accounting records. Sequent files every record in an audit-proof, immutable way.