The clock is ticking for Purolator and the union that represents more than 8,000 of its workers to come to a compromise.
Those employees could walk off the job on Wednesday afternoon.
The two sides are at odds over contract negotiations where pensions and the outsourcing of jobs are said by officials with Teamsters Canada to be stumbling blocks.
Purolator has stopped accepting shipments until further notice.
That has many businesses scrambling to find an alternative courier until the company lays out what's next.
Teamsters Canada issued the 72-hour strike notice Sunday after the majority of its Purolator members voted to reject the company's final contract offer.
Purolator said in a statement that the company will try its best to deliver shipments already in its network, but delays could occur.
A mediation session was set to happen on Tuesday, with the aim of averting a strike.
Ian Lee, a professor at the Sprott School of Business in Ottawa, says a long labour dispute would cause plenty of short- and medium-term damage to Purolator.
He believes it could also be a boost for the company's rivals, as they look to scoop business from Purolator customers left in the lurch.
UPS Canada was reaching out on Tuesday afternoon to frustrated Purolator clients with offers of speedy service and a tidy small business discount.
@TechniweldCorp your business doesn't have to stop. Sign up for the UPS small business program & save 40%: https://t.co/CTwch8Y2Uw
— UPS Canada (@UPS_Canada) March 28, 2017
Lee says the potential of a long strike is a nightmare scenario for Purolator, its union workers, and for its parent company.
He says Canada Post is counting on a healthy courier business at Purolator to help lead the Crown corporation into the future, with a focus on catering to the country's booming e-commerce sector.
The strike deadline is 4:30 p.m. Wednesday.
(with files from The Canadian Press)