MPs are pressing delivery firm Hermes over how it treats its couriers after the company said it received 100,000 complaints from its workers last year.
Hermes drivers are classified as self-employed and are paid according to the weight of the packages they receive.
The delivery company told the BEIS committee it has been forced to pay back almost £52,000 to drivers in compensation.
Rachel Reeves, head of the committee, has written to the firm today asking why it does not provide its couriers with scales so they can easily check the weight of their packages.
Head of the Work and Pensions select committee, Frank Field, who co-signed the letter, also criticised the company for designating its workers as self-employed.
“This looks distressingly like yet another example of companies using a designation of “self-employed” which does not stand up to any scrutiny to shirk their responsibilities," he said.
Reeves said: "As a committee, we've heard some disturbing testimony about the day-to-day reality of life for couriers, of couriers being told by managers that parcels come before people. How confident can we be that the policies and processes extensively outlined by Hermes are being observed in practice?"
She said she had not seen evidence that Hermes could guarantee couriers were receiving the national minimum wage for their deliveries.
Hermes said in a statement: "Our response will also reiterate that our self-employed couriers enjoy flexible working and earnings well in excess of the national minimum wage.
"In fact we have set our minimum standard at £8.50 per hour, taking into account any expenses the couriers may accrue. We are confident in the accuracy of our courier pay model and our records clearly show that our average courier rate is £10.60, 41 per cent above the national minimum wage, after all legitimate expenses have been deducted.
"This has been verified using real time data gathered from a significant sample of rounds, supported by route modelling."