Click on Subscriber Lookup:

You can search by any of the parameters shown below and click the find button:

results appear under the find button:

*To search by country, you need to use the 3 digit country code that ACS uses. Use the 2 letter abbreviation for states when searching.  

Click Set as Current Customer to add their subscriptions to the cart:

They will appear in the cart and when you click to proceed to checkout, the customer number and details for the customer you chose will auto-populate in the fields.