A Microsoft extension to the ANSI SQL language that includes procedural programming, local variables, and various support functions.
If you want to get data for a specific date addtional to the date range, then it's a OR logic, not AND; don't forget the brackets =>
WHERE sof.Sales_Office = @SalesOffice
ands.postage = 'Y'
and (s.[Order Date] >= @FromDate and s.[Order Date] <= @ToDate
OR s.[Order Date] = '02/06/2017')