GROUP BY, CASE (easy) Problem

I am not to experienced with SQL and I guess the following is a piece of cake for most of you...
I want to get the sales amount for each shop, for each of three different products, and for the sum of the three products. The problem is that I only get for each shop for each of the product, but not the sum of the three products OR for each shop the total of the three products, but not for each product.

My query looks like this:

SELECT

Shop_name,

case

when prod in (1,2,3) then ‘milk_bread_butter’

when prod = 1 then ‘milk’

when prod = 2 then ‘bread’

when prod=3 then ‘butter’

else ‘none’

end as product,

sum(sales_amount) as sales

FROM

Dbo.sales

GROUP BY

Shop_name,

case

when prod in (1,2,3) then ‘milk_bread_butter’

when prod = 1 then ‘milk’

when prod = 2 then ‘bread’

when prod=3 then ‘butter’

else ‘none’

end



The problem is that I only get the lin 'milk_bread_butter' per shop. If I switch the order around in the case statement;

case

when prod = 1 then ‘milk’

when prod = 2 then ‘bread’

when prod=3 then ‘butter’

when prod in (1,2,3) then ‘milk_bread_butter’

else ‘none’



I get the sales amounts per shop for each of the products, but not for the milk_bread_butter total.



I would be very greatful for any guidance as how to solve this.

Thank you.

Best, Mal