While holding the Adoption Papers, the familiars are discounted to 10 coins. If the papers are dropped, they cost 15 coins.
Notes
A shop that has already been entered will not have its item selection changed, nor will dropping the papers revert them to normal.
In Greed Mode, only the shop pool item is replaced with a familiar and discounted.
This also applies to all shop pool items when playing as Tainted Keeper. Familiars spawned from the Treasure Room pool remain unaffected.
Familiars that appear in both the Shop pool and Baby Shop pool (Buddy in a Box, King Baby, etc) will be discounted by Adoption Papers even if the shop was visited before the trinket was held.
Flip: The ghostly items will be from the Shop pool.
Glitched Crown: While one of the items will come from the Baby Shop pool, the other 4 items cycled between will come from the regular Shop pool, all of which still discounted at 10 coins.
Keeper's Box: Does not affect Keeper's Box' items, meaning both familiars and shop items can be purchased in tandem over the course of a run.
Shops and 
Black Markets now sell familiars instead of the usual item selection.
Tainted Keeper. Familiars spawned from the Treasure Room pool remain unaffected.
Buddy in a Box, 
King Baby, etc) will be discounted by Adoption Papers even if the shop was visited before the trinket was held.
Steam Sale: Both of the discounts stack, selling familiars for 5 cents.
Chaos: Chaos takes priority in item selection. However, items will still be discounted.
Eden's Soul / 
Mystery Gift: Ignores the effect of Adoption Papers, spawning items from the Shop pool if used there.
Flip: The ghostly items will be from the Shop pool.
Glitched Crown: While one of the items will come from the Baby Shop pool, the other 4 items cycled between will come from the regular Shop pool, all of which still discounted at 10 coins.
Keeper's Box: Does not affect Keeper's Box' items, meaning both familiars and shop items can be purchased in tandem over the course of a run.
Member Card / 
The Stairway: The extra shops are unaffected by Adoption Papers.
Tainted Lilith Unlocks 
Mother Unlocks 
The Binding of Isaac: Rebirth
