According to what you posted, you have a relative date calculation, but nothing you are comparing it to. You need the date that pertains to the net sales variable you are trying to match to the -90 day relative date.
Depending on the type of prompt you’re using for your end date user response field, you will potentially only have the one date entered there, and even if it is a list, it will be a semicolon delimited string.
If you have a date object in the report that is specific to each record that goes into your net sales (like transaction date, order fulfilled date, etc. depending on business rules) you could use the relative date logic to narrow your net sales calculation to those dates of the required type falling within your 90 day range.