How to add up multiple selections from a slicer

I am probably showing my ignorance in this question expecting it to work when it doesn't :

I have a measure to lift Line Value from a SQL table Invoices

Sales Value = sum(Invoices[Line_Value])

I have Var in each further measure for

Reporting Week_TY / Reporting_Year_TY / Reporting Week_LY / Reporting_Year_LY

(where TY and LY are this year and last year respectively)

which has been suggested to me I change to measures too but that is a separate issue.

This then allows me to do further measures such as Example 1 to 2 where the only difference is whether I am calculating weeks weeks  this year or Last year in the 'Calc' bit. I'd like to to maintain my code integrity as I have other measures such as one for Profit that says - inevitably -

Profit = sum(Invoices[Profit])

so all I have to do is change my respective codes from [Sales Value] to [Profit]

Everything works swimmingly to calculate

Wholesale Sales Week This Year / Wholesale Sales Week Last Year

with a slicer for the year and week number which of course feeds in to

VAR Selected_Week = SELECTEDVALUE(Dates[Week_Int])

the problem is that the users now want to choose multiple weeks on the slicer so that if they choose weeks 15 / 16 / 17, say, the this year and last year are the sum of the chosen weeks for this year and last year

When I do it, this year is zero and last year is a sum which is not the combination

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//  Example 1

Wholesale Sales Week This Year =

VAR Selected_Week = SELECTEDVALUE(Dates[Week_Int])

VAR Use_Week = Selected_Week - [_Relative_Date_Offset Calc]
VAR Selected_Year = SELECTEDVALUE(Dates[Year Int])

VAR Reporting_Week_TY = Use_Week
VAR Reporting_Week_LY = Reporting_Week_TY - 1

Var Reporting_Year_TY = Selected_YEAR
Var Reporting_Year_LY = Reporting_Year_TY - 1

// The Calc bit :

VAR Hold_Calc = SUMX( FILTER(ALL(Dates),
Dates[Week_Int]= Reporting_Week_TY && Dates[Year Int] = Reporting_Year_TY),

[Sales Value])

Return IF ( Hold_Calc = BLANK(),0, Hold_Calc)

*************************************

// Example 2

Wholesale Sales Week Last Year =

VAR Selected_Week = SELECTEDVALUE(Dates[Week_Int])

VAR Use_Week = Selected_Week - [_Relative_Date_Offset Calc]
VAR Selected_Year = SELECTEDVALUE(Dates[Year Int])

VAR Reporting_Week_TY = Use_Week
VAR Reporting_Week_LY = Reporting_Week_TY - 1

Var Reporting_Year_TY = Selected_YEAR
Var Reporting_Year_LY = Reporting_Year_TY - 1

// The Calc bit :

VAR Hold_Calc = SUMX( FILTER(ALL(Dates),
Dates[Week_Int]= Reporting_Week_LY && Dates[Year Int] = Reporting_Year_LY),

[Sales Value])

Return IF ( Hold_Calc = BLANK(),0, Hold_Calc)

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