I've never messed with these services. However, there was a conversation on reddit a week or so ago (couldn't find it unfortunately) about these services being poorly implemented and poorly documented, and the conclusion was that Paypal was not worth the effort unless you had some overriding reason to use their ecommerce solution.
why don't use use open source free ecommerce software available on the internet? just think that my be a bit more trustworthy in view of customers and easier to maintain/update your site. It also help you handle things like shipping, emailing, order status updating and that way you could provide better customer services to your clients in a more organized way. Just my opinion though, some scripts are very complicated though. Better choose a script that is free to use and has an official support forum where you could seek help whenever problems occur. Believe, running a ecommerce store is not easy at first but one it's set up, you are all set. Save a lot of headaches in future.