Good Customer Discounts have been an available feature for a long time - I've been able to set them up for my personal shop and can see what stables have access to mine and what stable shops I have discounts to.
I went to a stable shop where I have a discount to and bought an item. It did not give me a discount.
Shop Discount - 30%
Item Price - 100 ebs
Discounted Price - 70 ebs
Final Purchase Price - 100 ebs
as an aside - if this feature gets looked into, can other Shop Features be looked at as well?
this includes:
the Trade feature - you cannot offer a trade unless you reach out to the stable through mail and then gift the item and hope the other stable gifts you the traded item(s). There isn't a clear transactional trade off. it all runs on the honor system (or getting a Mod involved) - LINK and small QoL - be able to link the Shop to the Stable - as is you cannot go to a Stable Page from their Shop Page