I am afraid that the subject of your posting - "exposing methods as RESTful services" shows that it would be good to read a little about the concepts behind RESTful web services before trying to implement one.
RESTful web services are not about exposing operations, it is about exposing resources on which you perform a very limited set of operations (usually GET, POST, PUT, DELETE).
Of course you can expose your method as a web service, but it won't be REST. In your case RPC would be a viable option: http://en.wikipedia.org/wiki/Remote_procedure_call
If you want to create a RESTful web service that creates orders, then you can POST the representation of an order, consisting of, among other things, a number of items, to, for instance, http://myserver/resources/orders.