How to show the Join with Crows Foot Notation
There ought to be another entity, whose key is StoreID. 
There is a many to many relationship between stores and customers.

In a simple ERD, you can just draw a line between the two boxes for Store and Customer and put a crow's foot at either end.  You can't put lifetime purchases in either of those two boxes.  So if you want to include that attribute in your diagram, add a diamond in the middle of the line you just drew.  

The diamond diagrams the relationship, and you can put lifetime purchases in there.  

When you go to design a relational database, all of the attributes will end up being stored in relational tables.