Paypal

<form id=”payment” action=”https://www.paypal.com/cgi-bin/webscr” method=”post”>
<input type=”hidden” name=”cmd” value=”_cart” />
<input type=”hidden” name=”upload” value=”1″ />
<input type=”hidden” name=”business” value=”PAYPAL_ACCOUNT_EMAIL” />
<input type=”hidden” name=”item_name” value=”STORE_NAME” />
<input type=”hidden” name=”custom” value=”CUSTOM_VARIABLE” />
<input type=”hidden” name=”currency_code” value=”USD” />
<input type=”hidden” name=”notify_url” value=”IPN_URL” />
<input type=”hidden” name=”return” value=”RETURN_URL” />
<input type=”hidden” name=”cancel_return” value=”CANCEL_URL” />
<input type=”hidden” name=”image_url” value=”LOGO_URL” />
<input type=”hidden” name=”rm” value=”2″ />
<input type=”hidden” name=”no_shipping” value=”2″ />

<input type=”hidden” name=”quantity_1″ value=”10″ />
<input type=”hidden” name=”amount_1″ value=”50″ />
<input type=”hidden” name=”item_name_1″ value=”PRODUCT NAME” />
<input type=”hidden” name=”item_number_1″ value=”PRODUCT ID” />

<input type=”hidden” name=”quantity_2″ value=”10″ />
<input type=”hidden” name=”amount_2″ value=”50″ />
<input type=”hidden” name=”item_name_2″ value=”PRODUCT NAME” />
<input type=”hidden” name=”item_number_2″ value=”PRODUCT ID” />

<input type=”hidden” name=”quantity_3″ value=”10″ />
<input type=”hidden” name=”amount_3″ value=”50″ />
<input type=”hidden” name=”item_name_3″ value=”PRODUCT NAME” />
<input type=”hidden” name=”item_number_3″ value=”PRODUCT ID” />

<input type=”hidden” name=”shipping” value=”10″ />
</form>

Reference:
https://merchant.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_Appx_websitestandard_htmlvariables
https://merchant.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_cart_upload
https://merchant.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_encryptedwebpayments
http://www.neo.com.tw/archives/1004
https://www.paypal.com/tw/cgi-bin/webscr?cmd=p/acc/ipn-info-outside