Also, what are some of the more payment schemes? There are some copmanies where credits will expire after one month and must be purchased again, others, a little longer, while others do not have an expiry date on credits purchased.
I think you have covered pretty much the "logical" ways to make that approach. I'd suggest you really decide the pros and cons of your implementation of the aforementioned methods, and decide on your own which one applies better to your intents.
On this "market", i have heard/read some experiences where the implementators end up being tagged as copycats due to the exagerated concerns in "keeping consistent", and thus making no effort whatsoever to be innovative, which proved to be ... a deadly shot in the foot.
\\pb
