Idempotent usage events: keys, dedupe windows and late arrivals
What a usage event's idempotency key should be made of, how long to keep it, and what to do with events that arrive after the billing period has closed.
Blog
Expert articles on EU billing, compliance and SaaS invoicing in the European market.
October 2026 12 posts
EU VAT OSS for SaaS: how the scheme works and what to store
How the One Stop Shop works for digital services, and which fields your billing system should store per invoice line so the quarterly return builds itself.
Product catalogue versioning: plan versions, snapshots, grandfathering
Why a plan shouldn't just have a price: plan versions, sales and contract periods, pricing snapshots on the invoice line, and meter versions.
DATEV export from SaaS billing: batch, documents, reconciliation
What goes into the DATEV posting batch, how postings link to invoices, and which three balances must agree at month end, with a worked Stripe clearing example.
Billing software for European SaaS: requirements and test cases
What to demand from billing software for the EU market, weighted must and should, and how to test each requirement yourself in a vendor demo.
Seat-Based Billing: Proration, True-ups and Seat Removals in Numbers
20 seats added on 16 April, 10 removed in May, 30 more on an annual contract: three seat cases worked through, with the rules your contract should state.
Choosing an API-based billing platform: what to test in a trial
Sandbox, idempotency keys, RFC 9457 error format, webhook outages, data export: a concrete two-week test plan for choosing a billing API.
Automating VAT ID checks: VIES, outages and proof per invoice
A validation workflow for EU VAT IDs in SaaS billing: normalisation, statuses for outages, retry rules, the German BZSt option and proof that matches each invoice.
SaaS Price Changes Without a Deploy: Versions, Grandfathering, Dates
From €99 to €119 per seat: what a new price means for monthly customers, annual contracts, grandfathered accounts and open quotes, and what breaks on the day.
Reverse Charge Invoice: Mandatory Details, Sample, EC Sales List
Mandatory details and their typical mistakes, the exact wording, an annotated sample invoice, VAT category AE in the XML and the EC Sales List check.
Validating XRechnung against EN 16931: schema, Schematron, rejects
Three validation layers, one validator, five recurring rule failures: how SaaS teams check XRechnung before sending instead of after the customer complains.
E-invoicing software for SaaS: German deadlines and a buyer checklist
Receiving since 2025, issuing from 2027 or 2028: which capabilities e-invoicing software for SaaS needs, when, and what to test in the vendor demo.
August 2026 14 posts
EU VAT mistakes in SaaS billing: eight failures and their fixes
A failure catalogue for EU VAT in billing: how each error shows up, where it comes from, how to correct it and which query catches it early next time.
Billing webhooks: signatures, retries, ordering and replay
Signature checks per Standard Webhooks, deduplication by message ID, ordering by object version, and a reconciliation job for missed events.
Billing API for European B2B: what the data model must cover
Tax profile, invoice snapshot, credit note, selling entity: the data objects where generic billing APIs break down in European B2B.
GoBD for SaaS billing: immutability, audit trail and eight years
Germany's GoBD principles applied to a SaaS billing stack: locking, versioned master data, deletion dates from year end, and process documentation that keeps up with releases.
IFRS 15 for SaaS: one contract worked through all five steps
One SaaS contract with a licence, a migration fee and API usage, run through the five IFRS 15 steps: allocation, January revenue and a mid-year upgrade.
Volume vs. Graduated Tiers: SaaS Tiered Pricing, Worked Through
One price table, two invoice totals: volume and graduated tiers worked through, with the boundary cliff, the counting scope and the rounding rule.
Overage Billing: Included Quotas, Excess Usage and Plan Changes
A plan with 5,000 included documents, worked through: what happens to the allowance on a mid-month upgrade and when the overage gets invoiced.
Evaluating usage-based billing software: a seven-test scorecard
A scorecard for usage-based billing: idempotency, late events, pricing models, traceability and EU tax, tested on your own data rather than vendor slides.
Peppol in your billing system: access points, participant IDs, status
Peppol is a delivery network, not a format. When SaaS vendors need it, how access points and participant IDs work, and what your billing system has to store.
XRechnung for SaaS invoices: the fields your billing data must supply
Seats, prorated add-on seats and API usage as an XRechnung: the fields your billing data model has to supply, and where the Leitweg-ID comes from.
Dunning for B2B Subscriptions: Default, Reminders, When to Pause
When a business customer is in default, what you can then charge, a dunning sequence day by day, and six cases where the automation has to stop.
SEPA Direct Debit for SaaS: Mandates, Pre-notification, Returns
What matters before, during and after collection: creditor ID, mandate evidence, refund windows, the B2B scheme, pre-notification and return codes.
Reverse Charge in Software Billing: Decisions and Credit Notes
A decision table, a state model for VAT numbers, one decision record per invoice line, and what must happen on status changes and credit notes.
EU tax logic in SaaS billing: the order of decisions
Five tax questions, one fixed order and a stored decision object per invoice line: how to build EU tax logic you can test and explain line by line.
July 2026 2 posts
Subscription billing for the EU market: data model and events
An architecture for EU subscription billing: data model, immutable events, the tax decision at invoice creation, and returned debits that actually reach the product.
Closing the OSS quarter: controls and corrections in billing
What to check between quarter end and filing the OSS return, and how credit notes and late VAT IDs should land in quarters you've already closed.