The trusted partner needs to be certified as PDP (Plateforme de Dématérialisation Partenaire). Their role will be to act as a trusted third party and transform, validate and send invoices to the French tax authority.
A good provider should also have experience in providing similar services in other countries, so you are sure they are exchanging your sensitive data securely, reliably and most of all in line with governments’ requirements.
The best solution needs to be in line with French requirements, so make sure the provider understands and is compliant with all requirements (for example, that they can technically serve the required format factur X).