Custom menu one action (PL/SQL) for multiple rows

Yes, you can (caveat, not 100% as I don’t know exactly what you are trying to do). You can use an ampersand (&) to pick up the value for each row e.g. &SUPPLIER_ID would use the Supplier ID on screen.

I see, I read that a bit too quickly. I don’t know of a way, but if we get a little bit creative it could probably be solved. I’m assuming there isn't a common key for these lines e.g. you want to send the quantities from a PO and we could just pass the Order No into a loop? 

If not, off the top of my head, you could probably have the code set a value on each of the lines (i.e. code sets custom field ‘send qty’ to TRUE) and then use that to identify which lines you should include in your email. Then you could clear them if this is a repetitive action. It would be a little bit convoluted, but I’m fairly sure that would work. 

DECLARE

   qty_list_txt_ VARCHAR2(4000) := 'The following quantities are on order';

   CURSOR get_qty IS
      SELECT buy_qty_due,
             part_no
      FROM   ifsapp.purchase_order_line_part
      WHERE  order_no = &ORDER_NO;

BEGIN
   FOR rec_ IN get_qty LOOP
      IF LENGTH(qty_list_txt_) < 3946 THEN
         qty_list_txt_ := qty_list_txt_ || CHR(13) || CHR(10) || ' ' || rec_.part_no || ': ' || rec_.buy_qty_due;
      END IF;
   END LOOP;
   IF qty_list_txt_ != 'The following quantities are on order XX' THEN
      Error_SYS.Appl_General('PurchaseReqLinePart', 'ERROR: ' || qty_list_txt_);
   END IF;

END;

I’m assuming that you are sending these emails externally i.e. not to another user in the system?

All I can think of is to toggle the Action Taken from ‘Action not taken’ to ‘Action taken’ and then back? It would be a tricky screen to solve this from.

That’s the one editable field on MRP Action Proposals I can think of, MRP_ACTION_TAKEN. 
Don’t know if it’ll work, but when you set it to ‘Action Taken’ it will also set an ‘Action Taken Date’ which you could then use to find the lines that you want to include in the email.