I was looking for exactly the same problem. Sage SDK is £ 1,500 a year! and my understanding after talking with Sage is that any applications developed in this version will not necessarily work with future versions of Sage, so you will have to spend ANOTHER !!! £ 1,500 to get the latest dll, etc., if your application needs to be updated for later versions. This is too rich for me and, more importantly, my client.
I searched for alternatives and found SDATA on
http://sage.imtqy.com/SData-2.0/
This is quite complicated material, but for everyone who is interested, it allows me to request information about clients, and, most importantly, it allows me to add and edit clients. I am going to try adding an invoice.
I used the link above and the .net client example here as a link
https://github.com/Sage/SDataCSharpClientLib
Hope this helps someone.
Paul cookson
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