Integrate with Sage50c

Hi folks,

Any advice would be greatly appreciated.
I'm just trying to determine the best approach to integrate with sage50c v25 for a friend.
Had hoped that they would be using Sage Business Cloud Accounting. From what I see, this would have been straightforward for me to integrate with using its well defined API. But now looks look Sage Business Cloud Accounting may not be suitable.

All I really want to do is insert a new invoice. Is the best approach to buy a 'connector' from a third party. E.g zynk. It seems quite expensive for all I am trying to achieve.

Or is there any API that be used sage50c?

Kind Regards