How do I get Sum of two Colunms with Same Foreign Key

I have three tables: Carrier(C), DropShipper(D) and ShoppingCart(S) with the following schema

Table C

c.id(Pk,int,not null)
c.dropshipperid(Fk,int,not null)
c.Prodid(int, not null
c.cost(money null)

Table D

D.Dropshipperid(Pk,int,not null)
D.Dropshipper(nvarchar(50))
D.Remarks(nvarchar(50))

Table S

s.cartid(PK, char(36))
s.prodid(pk,fk,int not null)
s.qty(int not null)

Here are sample data:

c.id    c.dropshipperid  c.prodid    c.cost
--------------------------------------------
1            1             11          100
2            2             11          200
3           3             11          80
4            4             11           70
5            1             6          212
6            2             6          312
7           3             6          412
8           4             6          512


D.dropshipperid   D.dropshipper        D.Remarks
-------------------------------------------------
1                   Airmail               10-25days
2                    DHL                  23-5 days
3                     Fedex                6- 10days
4                       UPS                 4- 5days

S.cartid           s.prodid              s.qty
------------------------------------------------
   xxxx                 11                  2
    xxxx                 6                   2

And here is my sql

SELECT     D.DropShipper, C.Cost, D.Remarks,( S.Quantity * C.Cost)    AS  SubCost, S.CartID, C.DropShipperID, 
S.ProductID, C.ProductID AS cProductid,  S.Quantity
FROM C INNER JOIN D 
ON C.DropShipperID = D.DropShipperID 
INNER JOIN S 
 ON S.ProductID = S.ProductID
 WHERE (C.DropShipperID IN (1, 2, 3, 4, 5)) AND
(S.CartID = @cartid)

This is a sample result of my qry:

Dropshipper     cost     Remarkd     Subcost     Cartid    Dropshipperid
------------------------------------------------------------------------
   Airmail       100      Text         200        xxxx            1
   DHL           200                   400         xxxx           2   
   Fedex          80                   160         xxxx            3
   UPS            70                   140        xxxx            4
  Airmail         212                 414            xxxx          1
   DHL           312                  614          xxxx            2
   Fedex         412                  814         xxxx            3
   UPS           512                  1024        xxxx             4

Here is what I need:

I don Not what the DropShipperID duplicated, ie I need just one set. Then SubCost should be the sum of subcost for each dropshipperID. some thing like this

DropShipperID   DropShipper    SubCost    etc
------------------------------------------------
1                Airmail          614
2                DHL               1014
3                Fedex              974
4                UPS                 1164