There is Apple, and then there are Apple Authorized Repair shops. An Apple Authorized shop will do warranty repairs just like an Apple shop. Plus, because they are independently owned, they can do "other" stuff that an Apple Store won't do.
I can't tell you that your local Apple Authorized shop will pop in an SSD.... but .... if you buy the SSD from them, and the machine was in for warranty work any way... you might find that they will install something they have sold to you.