Hello,
Method, like any QB 3rd party application is limited by what is provided by the SDK from Intuit. The ability to convert a Sales Order to an Invoice is one of those features that are not included in the SDK, hence it isn't available in Method.
However, we can customize a work around that would allow you to copy the line items from a Sales Order and place them into an Invoice. One thing to note with this alternative though, is that you wouldn't be able to mark items as ordered. Also in QB you wouldn't see the relation between the Invoice and the Sales Order as you would if you had converted the Sales Order in QB.