First create a formula something like this.
shared numbervar subtotal;
if {Sheet1_.Item } = 'SUBTOTAL' THEN
subtotal
else if previous({Sheet1_.Item }) = 'SUBTOTAL' then
subtotal := {Sheet1_.amount}
else subtotal := subtotal +{Sheet1_.amount};
then create a second detail line that will have the item column and formula column in it. The first detail row will be suppressed if the item is equal to SUBTOTAL, the second detail row will be suppressed if the item is not equal to SUBTOTAL.