Report Designer help

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Hi All 

I am trying to amend the invoices and credits by customer report so that it shows the carriage charge invoiced. 

I have added the expression to calculate the carriage charge which it has done but i notice that if the invoice has more than one line, it adds the carriage charge onto each line and i cant figure out how to stop that as it throws out the figures completely as only one lot of £450 was charged

Can anyone help????