Immediate rebate deduction allowed before applying the shipping income percentage; deferred rebate treated as non-deductible business expense. Immediate rebate paid at collection may be deducted from gross freight before applying the percentage-based tax under sec.44B because only the balance is remitted to the shipping company. Deferred rebate, paid under conference conditions requiring shippers' exclusive support, is characterised as a business expenditure incurred by the shipping company and is not allowable as a deduction from gross freight for that purpose.