Hi,
As Jerome said, we started working on a rest API for HikaShop 2 years ago. And that's still something we want to push after HikaShop 3. The fact is that to get a rest API for all the frontend capabilities, it required a complete overhaul of the cart and checkout systems. And that's something which will be done with HikaShop 3.
We're not abandonning the idea, we're doing things in the correct order in order to plan for the future and have a solid base.
Jerome was simply telling you that usually, to integrate an ecommerce solution with an invoice software, you don't need a rest API in the ecommerce solution. Normally this is done by the development of an integration plugin between the ecommerce solution and the invoice software. Even if HikaShop had a rest API, it wouldn't change the fact that you would still need such plugin to be developed.
If your potential client wants a rest API plugin for HikaShop, then it's not available (although it can also be developed on your end if necessary). If he wants an integration with an invoice software, then either there is already such plugin (like for KashFlow or QuickBooks), or a plugin can be developed in a similar manner.