I think this question falls under a different category than this one, but to answer your question:
I am currently testing with WP Simple Paypal Shopping Cart.
or for more info:
Some stats from the wp plugin page:
Requires: 2.8 or higher
Compatible up to: 3.5
Last Updated: 2012-12-8
17 of 24 support threads in the last two months have been resolved.
2 people say it works (latest plugin version v3.4 with wp 3.5)
So it seems stable, hopefully it fits your purpose.