SUM in multi-currency

I am trying to do SUM() in a multi-currency setup. The following will demonstrate the problem that I am facing:-

Customer
-------------------------
Id | Name
1  | Mr. A
2  | Mr. B
3  | Mr. C
4  | Mr. D
-------------------------

Item
-------------------------
Id | Name   | Cost | Currency
1  | Item 1 | 5    | USD
2  | Item 2 | 2    | EUR
3  | Item 3 | 10   | GBP
4  | Item 4 | 5    | GBP
5  | Item 5 | 50   | AUD
6  | Item 6 | 20   | USD 
7  | Item 3 | 10   | EUR
-------------------------

Order
-------------------------
User_Id | Product_Id
1       | 1
2       | 1
1       | 2
3       | 3
1       | 5
1       | 7
1       | 5
2       | 6
3       | 4
4       | 2
-------------------------

Now, I want the output of a SELECT query that lists the Customer Name and the total amount worth of products purchased as:-

Customer Name | Amount
Mr. A         | Multiple-currencies
Mr. B         | 25 USD
Mr. C         | 15 GBP
Mr. D         | 2 EUR

So basically, I am looking for a way to add the cost of multiple products under the same customer, if all of them have the same currency, else simply show 'multiple-currencies'. Running the following query will not help:-

SELECT Customer.Name, SUM(Item.Amount) FROM Customer
INNER JOIN Order ON Order.User_Id = Customer.Id
INNER JOIN Item ON Item.Id = Order.Product_Id
GROUP BY Customer.Name

What should my query be? I am using Sqlite