Okay...I'm finding this very confusing.
So, officially, I can limit the number of times someone can use the skill before paying? (Once I've checked the box?) For example, I can direct the user to a 3rd party web site, on which they can 'pay with Amazon' to purchase the skill/or purchase a certain number of invocations?
Thanks April and Jo! You are both correct, developers are free to monetize their skills, some of the skills already doing it were mentioned by April, but you have to make sure to check the box Jo mentioned when submitting for certification.
Amazonians have said monetization outside of your skill is fine, and they've said it in multiple venues: in the weekly office hours chat Dave Isbitski hosts, in Q&As during webinars and at in-person events, and if I'm not mistaken, right here on this board.
I see no reason why Amazon would have a problem with it. There are already skills/features that require spending real money outside of the skill, like Pandora integration (which only works with premium accounts as of this writing, and that means users must have a credit card on file with Pandora) and Dominos (which requires you to already have a credit card on file and a boilerplate order saved to the Dominos site).
I don't think it matters where the database is.
What I think matters is if someone can incur costs through using your skill. There's a checkbox on the submission form that asks "Does this skill allow users to make purchases or spend real money?". What the consequences are of checking this is not made clear. I don't know if it is an automatic rejection, or if it triggers some other sort of examination. I am not aware of any skill that does let you make purchases or spend money.
It's also a bit vague. StarLanes, for example, has a virtual currency which you can spend in the game. When demand is high enough, I will be selling this for "real money". It's not clear to me if this then means that spending virtual currency in the game now counts as "making purchases with real money" or not.