An Evri courier has blamed senior management for ongoing delivery issues in Suffolk.
The employee, who has asked to remain anonymous, said drivers are having to work around the clock to tackle a backlog of parcels and claimed it was caused by the company "taking on more than they can handle".
The company says it has taken on extra workers but the delivery sector has been hit by staff shortages and sickness.
Evri has faced fierce criticism in recent months, with people across the county receiving orders late or not at all.
"I understand the frustration," said the driver.
"Some of them haven't had deliveries in months.
"But this isn't through a lack of trying.
"There is such a huge backlog and we're doing our best to get through it. We're mentally and physically exhausted."
The courier said many customers are told wrongly that drivers have their parcels four or five days before they are collected.
These problems have subsequently led to drivers receiving abuse from frustrated shoppers.
"I do understand why they're upset - especially at Christmas," said the driver.
"But it's not in our control.
"We're working around the clock to get this sorted."
The couriers claimed drivers are struggling to meet quotas the company has set them due to the sheer volume of orders.
This has led to a number of employees working as many as 27 days in a row and having their monthly bonuses wiped.
"Some of us are even having to cancel days off," said the driver.
"But we can't work any harder.
"But we will get everything done by Christmas Eve - no matter how long it takes."
A spokesman for Evri said workers' wellbeing was a "top priority" and "bonuses are not related to backlogs and have not been withheld".
He said: "Our couriers are self-employed and are free to choose the number of parcels they want to deliver, in which time-frame and to organise their days off.
"Some of our couriers decided to deliver on Sundays during this peak to get extra revenue but this has always been optional.
"We have taken on an additional 5,000 couriers this peak and have been actively recruiting in the area since October but like others in the sector we have been impacted by staff shortages and sickness.
"Whilst we are experiencing record Christmas volumes due to the Royal Mail strikes and have suffered some delays due to bad weather, our local teams are confident that all parcels will be delivered in time for Christmas."
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