If the shortcode was giving you trouble, it may be because of another plugin or your theme causing trouble with the parsing. I'd look into that to find which is causing the trouble.
You can change to the default theme and see if the shortcode doesn't give that problem anymore. If it continues, try deactivating other plugins one by one until the problem goes with it. Then ask the developer if he could fix that, or find an alternative.
The full code is not the best solution because it can be edited by the user, unlike the parsed shortcode if you have encryption enabled in your options.
WP Admin -> s2Member -> PayPal Options -> Account Details -> Enable Button EncryptionA better solution, if not using the shortcode, would be to create the button over at PayPal following Jason's instructions.
viewtopic.php?f=36&t=309#p1333I hope that helps.