I don't think you need the line items join table. I would put a _fkProduct ID or the orders table to link to __pkProducts ID. The __pkOrders is unique to each order-each order is a record of one customer, with multiple products. Each customer record is one customer with multiple orders.
So a portal on Customers can show related orders. A portal or products could show orders related to this product, or customers who purchased this product (2 portals?).
When you want to create a new order, you create a new record, then populate the customer, then populate the products
Sorry Steve but that line items table is needed. If you look at the Invoices starter solutions that come with FileMaker 12 and 13, these tables are called invoice data and are necessary to make up orders of multiple products that each link to the correct product record.
First detail: In this type of set up, one normally selects the "Allow creation of records via this relationship" for the LineItems table in the orders to lineItems relationship. This makes it possible to put a portal to Line Items on your orders layout and then be able to create new line items records by entering data into the blank "add row" that will appear in this portal.
Next detail: as a beginner's level start, you can then put _fkKardexID in the portal row and format it with a drop down list of values with a "use values from a field" value list. __pkKardexID would supply values for the first field and a product name or description field would be selected for field 2 so that you can use this value list to select a product by name/description, but the value list enters the ID number and links that LineItem record to a product record.
THis then makes two useful things possible:
a) data, such as a unit price can be copied from Products into a corresponding field in LineItems using auto-enter field options on that Lineitems field.
b) Fields, such as a name or description field, from Products can be included in the portal row to display more information about the selected product.
It works perfect, just as you described.