If it’s a discount on the cost of crafting nonmagical items, I’m much more ok with that. It’s easy enough to assume the character just has access to bstter sources for the raw materials they need than an amateur craftsperson would. If it’s on any nonmagical items they buy, which is the impression I got, that’s some nonsense IMO.
Then the ability should be rendered that way. Instead of just saying you get a 20% discount on all nonmagical items, it should say you can purchase lower-quality nonmagical items for 20% less than the cost of a standard example of the item (and spend some amount of time, maybe “during a long rest” or something) to restore them to the same equivalent quality of a full-priced example.