Entity Reference - am I using it correctly?
One way to do it is with Views and a module called Eva (Entity Views Attachment).  Eva allows a view to be attached to a host entity, and understands how to filter the view based using the host entity's id to display only related information.

Since you already have the entity reference field on the credit cart type, the steps would look like this:


Install Eva and Views as needed
Create a View for the credit card content type
In the view, add an "EVA" display
Configure the view to be attached to the customer (under Entity content settings)
Add a Contextual filter using the field that links back to Customer. This will filter credit cards using the customer profile entity