You can achieve this by Webdynpro modification. Find which Webdynpro is being used. You can just right click on the window where you see Drop down for Material and Service and go into More Field Help to find out. You get View also which in that screen. For example if i am using Operational Purchasing Role to create Shopping cart then adding Item With Free Description Webdynpro is /SAPSRM/WDC_DODC_SC_I_DES with view V_SC_DOFC_I_DESCRIBE, Please find in attached screen shot.
Once you get view go into T.Code SE80 and go into Webdynpro --> Views.
Open view in and in there Tab call Methods. You can do Pre or Post exit in the Method WDDOMODIFYVIEW to modify the view. In there you can clear the current drop down with first as Material and then Service and re change it to something like this;
ls_value-key = '01'.
ls_value-value = 'Service'.
APPEND ls_value TO lt_value_set.
ls_value-key = '02'
ls_value-value = "Material"
APPEND ls_value TO lt_value_set. where LS_Value and LT_Value as
DATA: lt_value_set TYPE wdy_key_value_table,
ls_value TYPE wdy_key_value.
Please have your ABAP or Webdynpro developer to enhance this as this code entirely will not work.