yes, this can be done, but not the way you are describing unless the shipping cost cost where to change from item to item, but then you wouldn't be using a flat rate anyway.
create a new table for siteSettings:
settingID - primary key
settingName - varchar
settingValue - varchar
in the database, create a record and set settingName to typeAShipRate and set the settingValue column to 7
then create a recordset to lookup the typeAShipRate
SELECT * FROM siteSettings WHERE settingName = 'typeAShipRate'
the use the set session value behavior to store the ship rate in a session variable. On the server behaviors panel, click the plus button and select eCart -> General -> Set Session Value
set the trigger to settings recordset not empty. set the name to typeAShipRate and for the value click the lightning bolt and select the settingValue column from the recordset.
the in the rule set the calculation to;
Calculation: Flat rate '$_SESSION['typeAShipRate']'