Unique Invoice Numbers Across Multiple Macs

The problem

You write invoice 2026-042 on your MacBook and send it. Before the sync reaches the iMac, the next invoice is created there — and gets 2026-042 as well. Rare, but unpleasant: invoice numbers must be unique by law, and the PDF is already with the recipient.

The solution

Just before assigning the number for good, GrandTotal asks a small reservation server hosted by Media Atelier whether it's still free. If not, it jumps to the next available number automatically. The request takes milliseconds — you won't notice.

What gets transmitted

Only the document identifier and a SHA-256 hash of the document type and invoice number. No customer data, no amounts, no line items. Reserved hashes are deleted after a few days.

Document: 6b7a4e9c-2f31-4d8a-9b6e-0c1f2a3d4e5f
Hash:     f882c38afe83ba7b929d8ad14a302055…

Neither the number nor the document type leaves your Mac. Both are folded with the document identifier into a single SHA-256 hash — the server only sees that string and cannot derive anything from it.

Opt-in and fallback

The feature is off by default: when the check is enabled, GrandTotal briefly contacts a server at Media Atelier on every send — so you know why this happens and turn it on deliberately yourself. It's enabled with the Check uniqueness online option in the settings under Invoices or Estimates — right where you also configure the number format. If the server is unreachable, GrandTotal falls back to local numbering — no blocking, no error.