NetSuite offers sandboxes to its customers, but they are not self-serve. You will need to contact your NetSuite account representative to request a sandbox. Once it has been provisioned, you can connect it to your Routable sandbox account and then develop against the sandbox integration to see your changes reflected in the NetSuite sandbox. As the sandbox needs to be requested by the NetSuite administrator within your company, they will need to grant access to the developer(s) working on the Routable integration to ensure that the team can see the results of the Routable actions being taken.
You can request that your chart of accounts, items, posting periods, and other settings from your production NetSuite account be copied to your NetSuite sandbox. This is called a refresh. It will be done automatically on provisioning of a new sandbox, but if you already have a sandbox that you haven't used in a while, it might be worthwhile to perform a refresh to ensure you're working the latest data set.
Updated about 1 month ago