the nature of sin

[Drich]

upon receiving confession isn't it the job of the priest to offer contrition or pentance for the sins confessed?

Bucky when he was a catholic school boy:
Bless me father for i have sinned, it has be 15 hours since my last confession.

Preist: what could you have possible do in 15 hours?

Bucky: I viewed and masturbated to gay donkey porn, then fashioned a large donkey 'wood shop project' out of a tree branch and reenacted my fav scenes. 

Priest: 
bless you son for the confession of your sin your penitence shall be 10 our fathers 36 hail marries and burn your wood shop project.

here used the prayer is used as a punishment/way to pay for your sin.
pentance:
1 : an act of self-abasement, mortification (see mortification sense 3), or devotion performed to show sorrow or repentance for sin He did charitable work as a penance.

Your definition is correct - it is an act of ‘self’ abasement, and is therefore voluntary. ALL acts of penance are voluntary. The prayers usually said after confession are the outward expression of the inner feeling, hence the term ‘act of contrition’. Contrition is what you feel, penance is how you express it. 

But prayers are never ‘used as a punishment’, that’s just silly. It’s simply more convenient for both priest and penitent if the act of contrition takes the form of prayer. 

Boru

then why put a number on the amount of times a prayer had to be repeated? that is what makes it a punishment. contrition would see a man pray till he felt he earned penance. maybe bucky would have felt penance after just one prayer, but now as a means to earn his forgiveness he must pray 35 more times.

(this is all silly anyway as the bible says we can not earn penance or work our way to contrition.) rather we are to repent. Which can be a completely internal change with no external changes made.