Summarising

Hi

'Sales' is my fact table with approx 5.5M rows and the granularity is at the order item level. An order can have anywhere from 1 to 9999 lines/rows.
The carriage/freight cost for each order is recorded on the item level and as such, if an order has more than one line/row, the value is repeated.
To allow me to SUM the unique carriage cost per transaction I want to group by the 'Sales'[Transaction ID].
Additionally I have these filters

•  'Sales'[Order Status] = "Invoiced" - To remove cancelled or incomplete orders.
•  'Sales'[Order Carriage (GBP)] <> 0 - To remove the 300K rows without a carriage charge.

This is what I have come up with so far and it works.

Carriage =
VAR __InvoicedCarriage =
    FILTER(
        'Sales',
        'Sales'[Order Status]="Invoiced" 
        && 'Sales'[Order Carriage (GBP)] <> 0
    )
VAR __GroupByOrderID =
    SUMMARIZE(
        __InvoicedCarriage,
        'Sales'[Transaction Id],
        'Sales'[Order Carriage (GBP)]
    )
VAR __Result =
    SUMX(
        __GroupByOrderID,
        'Sales'[Order Carriage (GBP)]
    )
RETURN
    __Result

My questions are as follows:

1) Is there a better (more compact/efficient) way to write my formula?

2) I thought it was bad practice to filter a fact table - How else could I do this?