Note:
1 Customer can order N Invoice, right ?
Yes, the product is good, want more !!
1 Customer can order 1 Invoice ?
No, he can order more than 1 times !!
1 Invoice can come from N Customer ?
Sometimes yes, if you want to record when a couple buy thing together but I don’t want so No !!
1 Invoice can come from 1 Customer ?
Yes, record only 1 is fine
Invoice & Customer is non-identifying
Conclusion is
1 (Customer) :N (Invoice) non-identifying