And this:
Hebrews [11] And every priest indeed standeth daily ministering, and often offering the same sacrifices, which can never take away sins. [12] But this man offering one sacrifice for sins, for ever sitteth on the right hand of God, [13] From henceforth expecting, until his enemies be made his footstool. [14] For by one oblation he hath perfected for ever them that are sanctified. [15] And the Holy Ghost also doth testify this to us. For after that he said: 

So, Christ paid for our sins but we have to pay for them too? Doesn't that diminish Christ's ultimate sacrifice, that we can in anyway add to it?
      Christ's sacrifice is effective whether we avail ourselves of it or not. All the saints who are saved, or will be saved, will win victory because of it. You must accept it in order to be saved or forgiven. That is what you add, your free will.


The forgiveness of our sins is predicated upon our sincere contrition, yet...they are apparently not entirely forgiven as we will be punished nonetheless and can only mitigate this punishment by expiation in the form of indulgences, prayers, charity, etc. in this life plus a huge amount of suffering in the afterlife?

         Mortal sins are forgiven by sincere contrition, and eternal punishment is thereby remitted; temporal punishment is forgiven by penance and good works. There is no suffering at all in the next life for those who have repented of their mortal sins and done sufficient penance for any remaining temporal damage they caused.

Well, I need to readjust my concept of mercy I suppose. 

     It is a Mercy for God to create you despite all the sins He knew you would commit. A mercy to Redeem You despite it all. A mercy to grant you the Grace to repent despite your sinning again and again. A mercy to forgive your ingratitude for all of the following. A Mercy to forgive any presumption you may have in sinning with the expectation of confessing it.  A mercy to give you so many relatively easy ways to atone for mortal sins before you have a chance to even confess them, and repair the temporal damage of your faults after. Etc, Etc.

It was probably just a coincidence that the selling of indulgences coincided with the reign of some very corrupt popes
like Leo X(from the biggest family of usurers in Europe, the Medici).
Seems the best way to get people to buy get out purgatory certificates is to play up its horrors. 
"... absolve you ...from all thy sins, transgressions, and excesses, how enormous soever they be...and remit to you all punishment which you deserve in purgatory on their account and I restore you...to the innocence and purity, which you possessed at baptism; so that when you die the gates of punishment shall be shut... and if you shall not die at present, this grace shall remain in full force when you are at the point of death."
That's what the indulgence actually stated

Psalms 102:11-15

[11] For according to the height of the heaven above the earth: he hath strengthened his mercy towards them that fear him. [12] As far as the east is from the west, so far hath he removed our iniquities from us. [13] As a father hath compassion on his children, so hath the Lord compassion on them that fear him: [14] For he knoweth our frame. He remembereth that we are dust: [15] Man's days are as grass, as the flower of the field so shall he flourish.

Who is the author of the book?

Purgatory"; "Explained By The Lives And Legends Of The Saints", by Fr. F.X. Schouppe, S.J. Tan Books, Rockford, Illinois. This book received its IMPRIMATUR on October 11, 1893, so these writings of the Jesuit priest, F.X. Schouppe, are more than a century old. Fr. Schouppe collected and compiled the descriptions of Purgatory as recorded in the lives of saints and various holy people. Some of the saints are well known (St. Catherine of Genoa and St. Gertrude) while others are less familiar, e.g. St. Odilo, or, for example, Blessed Stephana Quinziani. The book is divided into two parts, "Purgatory: the Mystery Of God's Justice" and "Purgatory: the Mystery Of God's Mercy".

https://www.thriftbooks.com/w/purgatory-explained-by-the-lives-and-legends-of-the-saints_fx-schouppe/269892/#idiq=1142631&edition=1811154