Most of what has been passed along in this thread is not accurate.
This site has had similar discussions in the past. I am surprised how 'confusing' this is to many people, especially collectors. The
Canada Post website and their rate sheets make it very clear as to what the
Canada Post policy is for mailing letters.
If you walk into a post office with a single letter to mail (or a few single letters), you will be charged $1 and a $1 stamp will be applied by the clerk (new $1 stamps in coil format were especially made just for this purpose). In this example a $1 stamp is applied to the envelope,
not a Permanent(tm) stamp with a face value of 85c that you have paid $1 for.
Here is the new $1 stamp showing the start of the roll. The stamps have been purposely spaced apart to make it easier to remove a single stamp. The gaps have even been slightly rouletted to allow for the excess backing paper to be removed easily.

GST/HST is
extra on any postal service, including this transaction.
This $1 stamp is applied by the clerk and your letter is kept by the clerk. You do not receive a $1 stamp to take home with you. I suppose you could ask for a $1 stamp, take it home, apply it to a letter and then return later to mail it, but if you take in a single letter for mailing it is kept by the clerk.
If you take in a single letter that already has postage affixed by yourself, you only need 85c worth of postage affixed to the envelope (whether it be a Permanent(tm) or any combination of other mixed-rate stamps). No need to go the counter, just drop it off in any post office box (but you can hand it to the employee at the counter, if you wish - I usually do and I ask for a hand cancel).
If you take in a single letter that has a 63c stamp already affixed, you can buy a single 22c stamp to make up the difference to 85c.
I suppose you could go into a post office and ask for three 25c stamps and one 10c stamp, walk away from the counter, apply the 85c postage to your letter, and then return the letter for mailing. You would save 15c on the mailing but (for many) you would have spent more in your time. What would be the purpose? To trick
Canada Post into mailing your letter for only 85c?
If you buy Permanent(tm) stamps in booklet, souvenir sheet or rolls, you are paying 85c per stamp (but you must buy the entire unit ... i.e. a full booklet of 10 or 30 stamps, full rolls of 100, 300 or 500).
[If you are a Venture One customer, you can now buy Permanent rolls of 300 or 500 and receive a 5% discount.]
Business customers with postage meters pay 75c per letter.
So, if you are a postal history collector there are now 3 basic rates to search for: a $1 mailing with the new $1 stamp; 85c mailing with stamp(s); 75c meter.
From my experience visiting post offices: I take in a single letter and ask for a stamp. In
every instance the clerk has said it is $1 to mail that letter
and suggested I could buy a booklet of 10 stamps at 85c per stamp.
Every clerk has given me the opportunity to save the 15c if I bought the stamps in 'bulk'. I have asked the clerks if people are buying the booklets to save the 15c per mailing. Virtually every clerk has said "no"; it seems people have no problem paying $1 to mail their single letter.
I liken this pricing by
Canada Post to supermarkets and other retailers. If you are willing to buy in quantity you can save a bit of money.