Multiple totals on matrix

hi, Anonymous 

I'm afraid that add an additional total row is not available for now.

And you may try this way to add a new custom row not a "total" row:

Step1:

Use this formula to create a new category dim table.

Dim CatName = UNION(SUMMARIZE(Account,Account[Category],Account[CatName]),ROW("Category",99,"CatName","Total Cost"))

Step2:

Create a relationship with Account table

Step3:

Adjust your formula as below:

Original = 
VAR Billing = Calculate(
Sum('Account'[Amount])
,Account[Category] = 0)

VAR Categories = Calculate(
Sum(Account[Amount])
,Account[Category] <> 0)
Return
IF(SELECTEDVALUE('Dim CatName'[CatName])="Total Cost",CALCULATE(SUM(Account[Amount]),FILTER(ALLSELECTED(Account),Account[Category] <> 0)),
 ABS(Billing - Categories))

Result:

By the way, you could use sort by column to sort the column order

https://docs.microsoft.com/en-us/power-bi/desktop-sort-by-column

here is pbix file, please try it.

Best Regards,

Lin