A client asked us if it's possible to have the products on their website integrated with Sage Evolution so that when products are purchased online by users the stock values on Sage Evolution will be updated as well. The client would like this to be integrated with their existing WordPress site.
From what I've found so far, I don't think there's anything already available to use with WordPress except the SagePay through WooCommerce plugin. I'm not quite sure if this connects to a Sage client and updates the stock as well.
I found a Sage Evolution SDK that can be used but I doubt the client would buy with that kind of fees. There's also the SData api which I found here: Integrating with Sage Financial Software.
I'm not sure if you can use SData with Sage Evolution.
I would appreciate any help on this, thank in advance :)
As you've already noted, Sage Evolution has its own SDK and does not use the SData standard. Developers need to purchase a developer license and end-customers need to purchase a customer license.
I'd strongly recommend you don't go straight to the database as direct db writes could compromise transactional integrity and the Evolution database tends to change structurally quite frequently.
To use the SDK, you can't avoid the customer license fee but you can get a head start and avoid coding the integration by hand with Flowgear (www.flowgear.net). The Community plan is free and supports low transactional volumes. More about the connector at http://www.flowgear.net/pre-built/sage-evolution and https://developers.flowgear.net/kb/Node:Sage_Evolution
Disclosure: I work at Flowgear.