Ahhhhh... now I see what you're doing and why it's not working.

Any reason why you cannot use ShipTo form as a subform on the order form? If
you did that, your command button could be used to make the subform visible
so that entry could be done in it -- and you could link the subform to the
main form via the CustomerID fields so that the value would carry forward.

If you want to remain with the popup form, then you'll need to use the
OpenArgs argument of the OpenForms method to carry the CustomerID value to
the form, and then have the popup form read that value and write it into the
CustomerID field on itself.

If you want this method, change your button code to this:

' start of main form's button's click code
Private Sub Ship_to_Click()
On Error GoTo Err_Ship_to_Click

   Dim stDocName As String
   stDocName = "ShipTo Form"
   DoCmd.OpenForm stDocName, , , , , , Me![CustomerID]

Exit_Ship_to_Click:
   Exit Sub

Err_Ship_to_Click:
   MsgBox Err.Description
   Resume Exit_Ship_to_Click

End Sub
' end of main form's button's click code

Then, in the ShipTo form, use this code (change names as needed) in the
form's Load event:

Private Sub Form_Load()
Me.CustomerID.Value = Me.OpenArgs
End If