As you correctly state, you need an attestation whenever you leave the house for essential goods and trips that cannot be done remotely or be postponed.
First, check that your task cannot be done remotely, either over the phone or online.
Many post office, bank, and insurance tasks can now be undertaken online or via telephone, for services such as checking or changing your details, changing a delivery, checking up on a parcel, downloading documents, chatting with customer service etc.
However, if your trip here is absolutely essential and cannot be done remotely or postponed, you are permitted to go, even if your post office etc is more than 1km away from your home.
To do so you need an attestation exemption form which is available to print and download here (in French). There is an English version available and also an easy-read version with pictograms. There is also a digital version available via the TousAntiCovid smartphone app.
You must tick the second box on the form, which reads “Déplacements pour effectuer des achats de fournitures nécessaires à l'activité professionnelle, des achats de première nécessité dans des établissements dont les activités demeurent autorisées, le retrait de commande et les livraisons à domicile”.
The same rules apply for other errands too (bank, insurance office…), with the same box needing to be ticked.
You can also tick more than one box on a single form if you are leaving the house to do several errands at once (such as visiting the post office and supermarket) or several errands under the same heading (e.g. post office and bank).
This is actually recommended, as it will mean you make fewer journeys outside your home overall.