The most basic interface into PayPal is the Buy Now button. Buy Now buttons can be used in a broad set of situations including: single item purchases, donations, payment for services rendered, custom eBay checkout, shopping cart checkout, etc.
The two required form fields car "cmd" and "business":
<form method="post" action="https://www.paypal.com/cgi-bin/webscr">
<input type="hidden" name="cmd" value="_xclick">
<input type="hidden" name="business" value="pay@site.com">
<input type="submit" value=" Pay ">
</form>
Payments typically include "item_name" and "amount":
<input type="hidden" name="item_name" value="Test Payment">
<input type="hidden" name="amount" value="0.25">
By default, payments are in US Dollars. "currency_code" can be used to specify other currency amounts (USD, CAD, GBP, EUR and YEN):
<input type="hidden" name="currency_code" value="CAD">
Options such as size and color can be specified with on0, os0, on1 and os1:
<input type="hidden" name="on0" value="Size">
<input type="hidden" name="os0" value="Medium">
<input type="hidden" name="on1" value="Color">
<input type="hidden" name="os1" value="Blue">
PayPal provides pass-through fields such as "item_number" and "custom":
<input type="hidden" name="item_number" value="PROD-123">
<input type="hidden" name="custom" value="123|abc|789|xyz">
