I do not agree at all with this last remark: Reseller accounts are for resellers, which clearly means that the reseller receives the invoice et makes a new invoice to his client. The client does not even know (that's up to the reseller) about Gandi. That's the way all reseller accounts work (and I have more than one with other companies).
It is exactly the way that accounts operate at the ICANN registrars I'm with and it is exactly what my clients want, and the reason is exactly that, domain ownership. An invoice and payment at an ICANN registrar are much stronger proof of ownership then any contact details in some form or a reseller without ICANN accreditation blocking off because they are the icann payer/invoicee.
Nevertheless, I think that what you want can be achieved with all reseller accounts: just creat one handle for your client that you will use as the "billing contact" for all domains he owns and that's it. As mentionned by support, when you create a new domain with a reseller account, Gandi's interface proposes you to use the reseller's handle as a contact for proprietary/admin/technical/billing contact, but allows you to change that at creation time: a specific owner, and/or a specific handle for each type of contact.
You keep on asking to create handles. Boy you must have fun managing god knows how many logins and passes!!
I must say that the Gandi's reseller interface is one of the best I have ever seen, and Gandi entering the hosting market is a good news for me, because of their very good service history as a registrar (I agree it's a different problem, we'll see). Pierre.
Then tell us Pierre, which are the reseller interfaces of ICANN registrars that also require you to log in with different log and pass for every domain owner when the domain owner contact details change??? I'll give you the heads-up on 2 that don't need different logins and passes for every owner like Gandi does: Godaddy and Moniker.