A series of good old "If" statements whilst perhaps not the most perfect solution would probably suffice ie
If PurchaseOrder > 10000 Then
' Generate email to Manager Level I
' Generate email to Manager Level II
' Generate email to Manager Level III
ELSE
If PurchaseOrder <= 2000 THEN
' Generate email to Manager Level III
ELSE
IF PurchaseOrder <= 5000 THEN
' Generate email to Manager Level II
ELSE
' Generate email to Manager Level I
ENDIF
ENDIF
ENDIF
Cheers