I understand how to put the document into the Doc Center - that really isn't the issue.   It would just be helpful to be able to keep all of my documents for one customer actually with the customer, and not in an enormous list of documents to have to scroll through in the Doc Center.    In the meantime, I found somewhat of a workaround to make things a little easier:

1.  I attach the documents from the Doc Center to my Customers as my vendor invoices come in.   This way it empties out the Doc Center so I don't have to scroll through 100's of attachments for various customers.

2.  Then, let's say 30 days later, I'm ready to bill my customer for all of the materials so far into the job, I go into the Customer attachments, detach them (let's say there's 4 attachments for that customer), and by detaching them, it puts only those 4 attachments back into the Doc Center list.

3.  I then create my Invoice to my customer, click on Attach Files, and I'll only see those 4 documents ready to attach to my invoice.

It's a bit cumbersome, but at least it's a workaround.   It's too bad there isn't a "Move" option to move the attachment from the Customer over to the Invoice.  I'll submit Feedback online