Urban Airship looks like a great all-in-one solution if you’re interested
in doing push notifications and in app purchases but don’t want to run your own
server. Their product pages are heavy on the marketing-speak, but it looks like
a solid product with a decent library and API.
Beware, though. As if depending on Apple isn’t enough, now you’re putting a
pretty important user engagement process in the hands of a third party. Weigh
that carefully. I’ve recommended this to some clients, though, because it was
simpler than having to support their own servers.