From BBPSOCO01 Custom mail to requester on 'Complete Item' button

hi all,

My requirement is when the buyer click the Complete Item button in SOCO ystsme should prompt the buyer to enter reason of rejection and include that reason in the mail and send it to the requester.

I am trying

1) TO stop the Standard mail going to requester. Is it possible?

2) Pop up a window from BBP_DOC_CHANGE_BADI OR BBP_DOC_CHECK_BADI, so buyer can enter the reason

3) incorporate it in the mail and send it to the requester

Can anybody help me in achieving the step 1 & 2. I tried to call FM FOBU_POPUP_GET_VALUE popup from these 2 BADIs but the POPUP is not coming in the ITS.

Any help will be appreciated

Thanks

Smita