An Item for 15-30 cents. Items will be from the Secret Room, Shop, Devil/Angel Room, or Boss Room items; there can be multiple items from different item pools, but items will stay to their chosen item pool if rerolled.
Devil/Angel room items can cost up to 50 cents, and Secret Room items can cost up to 99 cents.
The trapdoor spawns even if the shop was replaced by Greed or Super Greed, in which case it won't open until Greed has been defeated.
Similar to Crawl Spaces, there is only one secret shop per floor. The trapdoors in extra shops created by Red Key will lead to the same secret shop.
If the Member Card was removed from the player's inventory (for example, by playing Tainted Isaac and replacing it with a different item), the trapdoor will still remain open once it has been spawned, even after leaving and reentering the shop.
The trapdoor will appear in the Mirrored World version of the shop. Despite the shop itself being empty, the trapdoor will lead to the same secret shop where the items are still for sale. Buying from the secret shop in the Mirrored World will thus remove the bought item from both worlds.
It is likely a reference to the Member Card from the Fire Emblem series. In Fire Emblem, having a Member Card gives one access to unique and valuable items in shops, similar to the Member Card in Repentance.
Bugs
Bug!
Using XVI - The Tower? in the underground Shop can spawn rocks under the ladder, resulting in a softlock if Isaac can't fly or doesn't have any bombs.
Bug!
Using IV - The Emperor? in the underground Shop on Mausoleum II, and getting Rotgut as the boss, will result in an infinite loop of entering and exiting the boss room and the shop, respectively.
Bug!
On Downpour II (or one of its alternate versions) :
Don't go into the underground shop.
Go in the mirror world, enter the underground shop's alternate mirror version. It should have nothing for sale.
Make the game refill shop. For example by using a restock machine.
Go to the regular underground shop in the normal non-mirror world.
Now the underground shop contains two overlapped sets of things on sale : One from when the mirror version was restocked, and one from when you entered the "real" underground shop for the first time.
Use Flip to swap to the other form of Tained Lazarus.
Spawn and enter a Crawl Space, Tainted Lazarus will spawn in the center of the room.
Upon exiting the crawl space Tainted Lazarus will be placed in the Member Card shop, but will be softlocked as the exit ladder will now place him back in the Member Card shop.
Shop that leads to an additional underground shop that can sell 2-5 of the following:
Black Heart/ 
Bone Heart for 8 cents.
Eternal Heart for 15 cents.
Rotten Heart for 5 cents.
Greed or 
Super Greed, in which case it won't open until Greed has been defeated.
Crawl Spaces, there is only one secret shop per floor. The trapdoors in extra shops created by 
Red Key will lead to the same secret shop.
Tainted Isaac and replacing it with a different item), the trapdoor will still remain open once it has been spawned, even after leaving and reentering the shop.
Eden's Soul/
Magic Skin/
Mystery Gift: Only shop pool items are created if used in the underground shop.
Restock: Items bought in the underground shop will also be restocked.
Bug!
XVI - The Tower? in the underground Shop can spawn rocks under the ladder, resulting in a softlock if Isaac can't fly or doesn't have any bombs.
IV - The Emperor? in the underground Shop on Mausoleum II, and getting Rotgut as the boss, will result in an infinite loop of entering and exiting the boss room and the shop, respectively.
Tainted Lazarus :
The Binding of Isaac: Rebirth
