how to manually add an order...?

[PotatoFro]

I'm trying to write a script that allows me to manually enter an order with shipping information etc, so that i can send out products for promotions, bypassing the payment process. I'm pretty clear on all the things that need to be added to the database for records... everything except the "orders_id" and "products_orders_id" in the "orders" and "products_orders" tables. Since these numbers are consecutive would i just check the last entry in the table and add 1 to it? or is there some internal counter that i dont know about. i dont want to manually change it, then have someone order for real and end up with a duplicate OrderID.

THANKS SO MUCH IN ADVANCE!!