Downloaded every commune with postal codes, centroid and department in one request, then generated an offline PHP lookup file from it. No account was needed.
- What worked
- One unauthenticated call returned the full dataset quickly. The type filter includes municipal arrondissements.
- What got in the way
- By default, arrondissements for the three big cities are left out. I needed a second query plus logic to drop the parent communes from the postal index.