IT:AD:PayPal

* Advantages:

• Well established

* Considerations:

• http://www.screw-paypal.com
• PayPal Website Payments Standard does not require a Merchant account. They pay on PayPal, and get redirected back.
• PayPal Express Checkout requires Merchant Account. With it, they get redirected to PayPal, where they don't automatically have to sign in (if their cookie is valid).
• Rolling your own UI, or a Service, requires the Payflow Gateway.
  • Which requires a Merchant Account.
  • And it's more expensive to setup.
• Can create Subscribe and Unsubscribe buttons.

<A HREF="https://www.paypal.com/cgi-bin/webscr?cmd=_subscr-find&alias=65K84SYFZLKTQ">
<IMG SRC="https://www.paypalobjects.com/en_US/i/btn/btn_unsubscribe_SM.gif" BORDER="0">
</A>

<div>
<f.orm action="https://www.paypal.com/cgi-bin/webscr" method="post">
  <input type="hidden" name="cmd" value="_s-xclick">
  <input type="hidden" name="hosted_button_id" value="7WLSPJP94M3D2">
  <input type="image" src="https://dl.dropbox.com/u/11851202/USERS/SKYS/Resources/Purchase.png" border="0" name="submit" alt="PayPal">
</f.orm>
</div>

• Disadvantages:
  • Requires a US Address & Business to create a Business Account.

I was surprised that I did not find several libraries for working with PayPal. Only found the following:

• http://www.thecodenode.com/PayPalAPICSharpdotNetWrapper.aspx

• http://www.screw-paypal.com