The error is pretty straight forward:

you need to add a group by clause.

DECLARE @Amount MONEY;

DECLARE @totAmn MONEY;

SELECT ISNULL(SUM(p.price*s.quantity)+ s.shippingcost,0)

FROMtbl_shoppingcarts

INNER JOINtbl_productp ON s.productid =p.productid

WHERE s.cartid =@cartid

GROUP BY s.shippingcost;

Try to format your code. The person who has to read your code will be grateful.

You can also alias your tables, which makes your query more readable.