I found this to be a quality write-up with great examples.

While not a payment system, I've been implementing subtle animations to an app I'm working on and the improvement to the experience is immediately noticeable.

There's something about subtle animations — particularly when they come with their own, minute effects — that adds a lot of "quality" to an experience. For users, that quality equates to the value of the service and how much they're willing to spend to use it, either with money or time.