Simplecart.js is a free and open-source javascript shopping cart. It relies on your PayPal email address, so it would function similar to PayPal's Payments Standard but, without the PayPal Item button code. This cart basically uses the "upload" method to pass the only the end results of the shopping cart to PayPal. This is how most 3rd party shopping carts work.
Once the customer checks out, you will receive an email notification of the the transaction and you will also see a record of the transaction in your PayPal Account Transaction History.
If you need to update a database on your end or send an email from your system to the customer, then you will need to use PayPal's IPN feature. (IPN is also another way to notifiy you a successful transaction in addition to the email notification.) See the info below:
IPN Overview:
https://developer.paypal.com/webapps/developer/docs/classic/ipn/integration-guide/IPNIntro/
IPN Details:
https://developer.paypal.com/webapps/developer/docs/classic/products/instant-payment-notification/
IPN Guide:
https://www.paypalobjects.com/webstatic/en_US/developer/docs/pdf/ipnguide.pdf
