Ubercart need to pay depends on user role I am using drupal 6.20 and Ubercart 2.0
Like what I asked in the subject.
Is there any chance that I can customize a ordering process with or without payment?

For example:
There are 2 roles of users ( Role A and Role B)
Users of Role A need to pay all my products to complete the transaction.
Users of Role B don't need to pay any of my products to complete the transaction.

Actually I want my website to be suitable for 2 kinks of clients:
1. wholesalers (Role A) --- order online but pay through company invoicing system
2. retailers (Role B) --- order online and pay instantly

Appreciate any thoughts and helps on this!
This question has been solved and just want to put the solution here to help someone has the same interests!

Download the uc_purchase_order module, http://drupal.org/project/uc_po

And give the permission to the user role you want them to purchase through purchase order.