Hello Sir, Below is my plan:
1. I will create this membership number field on the cart page where customer can put their membership number.
2. On pressing UpdateTotals button on the Cart page, this membership number's first 6 digits will be cross checked to see if it is in the correct format. When customer places the order, this membership number will be attached to the order and saved in the database along with the order.
3. Admin can see this membership number on the Order's Admin page.
Kindly contact, am quite sure of doing this perfectly.