You'll need to get the CustomerID value into a field in InvoiceDetail so that a relationship from InvoiceDetail can match to CustomerPricing by both CustomerID and ProductID. If you only need this relationship to be "one way" (you only need to access CustomerPricing records from InvoiceDetail and never the reverse), you can define a calculation field in InvoiceDetail that simply lists the field name of your CustomerID field in the Customer table. If you need it to be "two way" (you also need to be able to access InvoiceDetail records from CustomerPricing), you should make this a number field that uses a Looked up value field option to copy the customer ID number into the InvoiceDetail customerID field.
One way is good for me. I'll give this a try. Thanks,