Canada Post says it has told the Canadian Union of Postal Workers (CUPW) that the contract offer it made late last month should be considered the mail service's "final" offer.

In a statement issued late Monday afternoon, Canada Post called the contract proposals presented to the CUPW on June 25 "fair and reasonable framework for settlements."

On July 1, the union presented a counter-offer, which Canada Post says is "not affordable" and would add $1 billion in costs over the term of the contract.

The union, meanwhile, is accusing Canada Post of preparing to lock out workers and creating uncertainty by warning the public to avoid the post office.

Earlier, Canada Post spokesman Jon Hamilton said the Crown corporation is hopeful the two sides can hammer out a deal, noting it still hasn't given the 72 hours' notice required to call a lockout.

The CUPW, meanwhile, has a strike mandate from its members, but also has yet to call for any job action.

That puts the timeline, if any, for a work stoppage at Thursday at the earliest.

Both sides remain at the negotiating table and Hamilton says he hopes they can reach an agreement that is "affordable and reasonable." The union, meanwhile, called the company's latest offer an "insult."

While many issues are on the table, the dispute ultimately boils down to money.

Changes to employee pension plans have been a key sticking point in negotiations. The company wants to switch new hires to a defined contribution plan, different from the current one which guarantees a reliable payout every month. The union doesn't want that and has also proposed some wage hikes.

A representative from Canada Post says in a written statement that the union's proposed plan would add "at least $1 billion" in costs over the next three years. 

The statement also says that two other unions representing Canada Post workers have agreed to the pension changes and only CUPW is holding out.

Canada Post is mired in a changing economic climate for its core business. While the company took in more than $8 billion revenue last year — an increase over 2014's level — the volume of mail being delivered has shrunk for several years in a row. Canada Post delivered 8.9 billion items last year, down from 9.1 billion in 2014, and a figure that would have been much smaller were it not for growth in parcel delivery partially offsetting a large decline in letter mail. 

The last time Canada Post experienced a work stoppage was in 2011, which included 10 days of rotating strikes and a lockout before the previous Conservative government mandated the company to end the dispute with back-to-work legislation.

With files from The Canadian Press