Things might have changed, but I have never seen a truly refillable cup anywhere, not even at WWS. In fact it was at WWS where I decided to use, and pay for the refill, my "souvenir" mug/cup, and they felt the need to be VERY clear that I wasn't getting a bit of a discount. Since I had told them "and the 2 cups of coffee", or something like that, I felt that was obnoxious (don't you hate when you state something up front but the person isn't listening?), but I paid the same for my cup as for hubby's disposable cup. While there I also determined that the mug/cup was the same number of ounces as the disposable cup. Just had to make sure!
Anyway, at a station where you are putting in the liquid you can surely use a souvenir cup, but you're not going to get a discount.