You can choose UPS when you are making your listing.
It is part of the dropdown menu of shipping services.
Purolator (which belongs to Canada Post*) does not have a contract with Paypal and is not directly shown on the dropdown menu.
You can use Calculated Shipping with UPS or Canada Post, but not with other shippers. Since rates are based on dimensions and distance as well as level of service this can lead to problem. Mostly losing money type problems.
Couriers tend to be more expensive than postal systems.
Couriers do not operate in most rural or isolated areas. Usually they sub-contract these deliveries. Usually to Canada Post.
*https://www.canadapost.ca/cpo/mc/aboutus/corporate/groupcompanies.jsf
