It's actually been going on for a bit longer than that. First time I remember getting the $1 in store discount was back in May. The trick for getting it is you have to select shipping when you checkout and during the checkout process it will eventually (but not always) offer a dollar discount to switch from shipping to in store pickup.
I don't have GCU, but that's pretty cool of them to offer. I would guess if it's not offered 100% of the time, it has to do with them saving funds on shipping, and in turn, gives the customer a break as well.
I have an Amazon center in town, but even if they offered the $1, I doubt I would bother, but it probably would help in regards to damages. I get bubble mailed items all the time that they deliver directly, and almost every time stuff arrives OK. If the same stuff came the same way by UPS or USPS, it would be damaged almost every time, go figure uh?