Regular mail will be much delayed. You might want to use Xpresspost service of Canada Post and send it to following courier address:
CPC-Sydney
49 Dorchester Street
Sydney, NS
B1P 5Z2
Eventhough CP xpresspost suffering from delays as well, I highly suggest you should use Purolator or Fedex to ensure delivery in 1-2 days by dispatching to same above courier address. Good luck!
I think it's better to include "Citizenship Grants" or "GRANTS ADULTS" as the second line whether it's regular or priority mail, it might help in quick mail sorting.