here's a more consise and complete desc for Karma in R+. <blah> are for icons. Note that the confessional has the same behavior as Blood Donation Machine, so you can have it simply replace any BDM icon with <BDM icon>, <confessional icon> if unlocked:

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Reduced chance of the Donation Machine jamming. [hide if greed mode].

Increases chance of Luck gain from <Donation Machine> donations to (x%). Adds possible bonus effects to <Donation Machine>, <Restock Machine>, <Blood Donation Machine>, <Confessional>, and <Beggar>. Possible effects are:

[if greed mode: Chance of one bonus effect from <Restock Machine>, <Blood Donation Machine>, <Confessional>, or <Beggar>. Possible effects are:]

+1 Luck <luck icon> (x%) from: (all)
Heal 1 <Red Heart> (x%) from: (all except restock) (hide on containerless characters, +2 heart charges for T.Beth [unsure, confirm please?)
Gain +1 <Penny> (x%) from: <Donation machine, Beggar>
Spawn 1 Beggar <beggar> (x%) from: <Donation Machine, Blood Donation Machine, Confessional>

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The wiki says that, golden/box/stacking Karma increases the +1 luck reward with a cap of +5, but that hasn't updated since R+ gave it a big overhaul. Need to check and see which of the rewards are amplified.