How do I get Store to list individual items and their prices on PayPal? At the moment, the only information which is displayed on the PayPal site is the order number.
Let me start by saying that PayPal will take your order subtotal and tax rate and want to calculate the tax amount on their own. Normally that wouldn't be a problem but PayPal is notorious for rounding the values wrong which will cause the transaction to fail as their tax calculation is different then what is passed through via Store. So you need to make sure that you run though more test transactions then you normally would to make sure that the rounding issue is not a problem with your site.
This is assuming that you are using Store 1.6.1:
While this is not officially supported (I am not sure when or if it will become officially supported) it should do what you need. CI Merchant GitHub Fork will be replacing the system/expressionengine/third_party/store/ci-merchant/libraries/merchant/merchant_paypal_base.php file with the one from the repo. From there you should be seeing your orders data being passed in.