Just a quick explanation of the difference between the PayPal services Website Payments Standard and PayPal Express Checkout and why you would choose one versus the other. I've found myself asking this question more than once now, so next time I can just recall what I wrote here.
Website Payments Standard is a completely FREE* PayPal service that can process credit cards without requiring a PayPal user account. The disadvantage is that all processing takes place off-site. That is, the integration with the retail site is limited to just re-directing the user to PayPal for checkout. After this happens, there's no guarantee the user will ever return to your site which makes having a consistent confirmation page impossible. But beggars can't be choosers and this service is, after all, free.
Express Checkout is also FREE* and offers better integration with your site. It works by directing users to the PayPal site just to enter payment info, and then returning to the retail site to complete the order. All in all this seems like a better solution, allowing complete control over the final stages of checkout, including payment processing and confirmation. Additionally, recurring payments (a.k.a. billing agreements) can also be setup and even combined with regular one-time purchases. The down side is that the API programming is more complex and the amount of user interaction required to complete an order can increase (a possible negative impact on conversion rates). But if you're comfortable with the free API (PayPal NVP) then I'd say the benefits far outweigh the disadvantages. Note that PayPal also offers a SOAP interface, but I believe use of this requires a paid account.
Either way, if you choose to integrate a FREE* PayPal service expect many headaches on the way. My experience with PayPal places it as #1 in the Worst Software Integration Experiences. Their developer/sandbox tools are more flare than function, their documentation is selective or just out-of-date (not sure which, but probably both), and their technical support is utterly useless. But what alternative do we have? For smaller volumes of sales, it's still (by-far) the cheapest credit card processing service on the planet. Have fun!
*NOTE: The word FREE here refers to no monthly service charges. All merchant gateways charge transaction fees with PayPal on the high-end.