Hi,
It would be 1 month from the start of the project. I suppose there would first be things to discuss regarding the implementation.
Like, which parts of the API will be implemented, what needs to be dynamic and what needs to be provided as a setting of the plugin.
For example, I suppose the main API will be the POST to invoices/draft to send the details of an order. This means the plugin will have to provide a vatZoneNumber for the shipping address of the user. So the question is how to provide this value ? What would be the different zones ? I suppose the plugin could have a setting to match the country code with a vatZoneNumber. But maybe you're in the USA or Canada and you want different vatZoneNumbers for different states ? And so matching the country code wouldn't be enough, which means it would also have to be at the state level, which complexifies the implementation, and thus requires more time and is more expensive.