RE: Get a list of open and recent invoices from a web service
If you want to know what invoices have been shipped and invoiced then you need to look at posted sales invoice (132) or Posted Sales Invoices (143), which is basically a result of sales order is posted and invoiced.
In Navision you can also create sales invoice directly which is basically a charge to the customer and once you post that even end up in Posted Sales Invoice. These open invoices which are not yet invoiced then you can get that information from Page 9301 or Page 43 (Sales Invoice)
In both scenarios you have posting date/Document Date which you an use to filter it.
Below is the link showing an example how to filter on fields.
msdn.microsoft.com/.../dd355316.aspx