Here are the rules the customers should be aware of when sending postmarking requests to Canada’s National Philatelic Centre in Antigonish, Nova Scotia, for FDC cancels:
You (anyone from anywhere in the world) can send a cover to the National Philatelic Center in Antigonish with a cheque for the cost of any stamps they are purchasing from them, plus a small service fee listed above. If you are purchasing a stamp to be affixed to your cover (which foreigners may need to do, if Canadian stamps aren’t readily available where you are), you have to pay the price of the least format in which the stamp is being sold. In case of a stamp that is available as a booklet of 10 ‘permanent’ stamps (currently worth of $0.99 CAD), that meant a $9.9 for one ten-stamp booklet, plus a $0.20 affix-and-cancel fee, and a letter explaining that you wanted only one stamp on the cover.
You don’t need to supply a return envelope with valid postage stamps, because the Centre sends the results back at their own cost.
FDC cancels can be used postally so long as the item bears enough of postage stamp(s). See, for instance, this postcard sent to Germany from the Centre:
Additional information about the rule highlighted in pink:
This rule states that your item cannot bear a previous cancellation – whether a FDC cancel or a regular dated cancel from a local post office. So for instance, for my eclipse lottery, originally I wanted to create a ‘double maxicard’ with first the dated cancel of Niagara Fall and second the FDC cancel:
My cards postmarked with Niagara Fall cancel were returned to me uncanceled from the Centre, due to this rule.
When I raised the point that my Ukraine cover was cancelled twice (see below),
Nicole of the Centre explained to me in an email:
The rule about 1 cancel has been in effect for as long as I have been doing this job.
The Ukraine covers you showed should not have been canceled a second time, but perhaps the staff who processed your order was not aware of this rule.
This rule should be followed across the corporation, even at stamp shows.
Nevertheless, it’s unsure whether this rule is supposed to apply to every cancel administrated by Canada Post (including the dated cancel from a local post office), or only to FDC cancels (*); and whether this rule only applies to one side of the cover, but not the other side.
For my eclipse request, I did get the postcard I mailed to myself cancelled again. This is the address side:
And on the image side, they did put the FDC cancel for me:
It’s not clear whether this was a mistake or it was allowed this way. 
(*) I am pretty sure that even if there is such a rule for ALL cancels administrated by Canada Post, it’s hardly ever been enforced in practice. Because I got my items cancelled a great many times all the times 