The On Checkout stored procedure doesn't pass any parameters.
The way that I work this is by adding another field in the Smith_StoreOrders table. In my case, it's called "SentToMas", but you could call it whatever you want.
Then, in your SP, select the orders where [SentToMas] = 0, and when you process each record, set [SentToMas] = 1.
I also have a field in there called "Printed", which gets updated after the order is rendered into a pick sheet and sent to the warehouse printer.
This way, if something goes wrong, your stored procedure doesn't fire, or has an error (for example, when I send orders over to MAS90, if the user has put in an invalid zip code, the procedure stops)... you can call your SP manually after the problem is fixed, and the orders "catch up".
So, all in all, it's a way to process ALL unprocessed orders, instead of a single order by passing an orderID or whatnot to the SP.