I did do that, and this is what I found. Notice that the url to paypal (at the very top) is a generic one. All the code that is needed to recognise the transaction in the html.
<form method="post" action="https://www.sandbox.paypal.com/cgi-bin/webscr"> <input type="hidden" value="_xclick-subscriptions" name="cmd"></input> <input type="hidden" value="utf-8" name="charset"></input> <input type="hidden" value="TipsandTricks_SP" name="bn"></input> <input type="hidden" value="xxxxxxxx@gmail.com" name="business"></input> <input type="hidden" value="USD" name="currency_code"></input> <input type="hidden" value="233" name="item_number"></input> <input type="hidden" value="Subscribe Now-test" name="item_name"></input> <input type="hidden" value="1" name="a3"></input> <input type="hidden" value="1" name="p3"></input> <input type="hidden" value="D" name="t3"></input> <input type="hidden" value="1" name="src"></input> <input type="hidden" value="1" name="no_shipping"></input> <input type="hidden" value="http://xxxxxxxx.com/?swpm_process_ipn=1" name="notify_url"></input> <input type="hidden" value="http://xxxxxxxx.com/test-sign-up/" name="return"></input> <input type="hidden" value="http://xxxxxxxx.com" name="cancel_return"></input> <input type="hidden" value="subsc_ref=3&user_ip=216.xx.xxx.142" name="custom"></input> <input class="swpm-subscription-button-submit" type="image" alt="Subscribe Now" src="http://xxxxxxxx.com/wp-content/uploads/2015/07/ancestral-check-out-button.png"></input> </form>
And just now, something else occured to me. Do you know if this code would work in a 'Mail Chimp' email that has been formatted with HTML?