What you need then is a way to tell frmReceive that it is being re-opened after a new customer was added, and that the combo box should be set to the new customer ID. You could use a Global variable for that.
Another way to do this would be to have the code behind your new customer button just hide frmReceive, rather than closing it, open the new custome form as modal (see below), and then unhide frmReceive, something like this (untested, but close I think)
Code:
'
' Hide frmReceive
'
Forms!frmReceive.Visible = False
'
' Open form for new customer in dialog mode, to pause THIS code until that form is closed
'
DoCmd.OpenForm "frmNewCustomer", , , , , acDialog
'
' Unhide the frmReceive form
'
Forms!frmReceive.Visible = True
'
' Requery the combo box to ensure new customer is included
'
Forms!frmReceive!cmbCustID.Requery
'
' Set the combo box value to the new customer
'
cmbCustID = DLast("CustNumber", "tblCustID")
This way you don't have to worry about communicating between forms quite as much.