You may need a link entity table.  For example if you're selecting customers
from a combo list, I'm assuming you're assigning this selection from
customers to the part numbers they require.  So MANY customers would link to
MANY Part numbers effectively, therefore you would have an intermediate table
to break up the many to many relationship.

This would mean then that the link entity table which could have the fields
relevant to the order such as date, part no. delivery details, contact name
for order, etc all in one table and you shouldn't get the error message.

The customer table should only then require information that would not
change, such as maybe invoice address, tel. no, company name.  

To apply this, you would have a one to many relationship from customer table
to link (order details) entity and then a one to many from part number to
link (order details) entity.  Essentially you're using the customer and part
number tables as look ups to populate the link entity table which is unique
for the order.

Hope that helps and I haven't misunderstood you're query!
TN