Solution:
PMPro developer here. The checkout page requires that ?level=1 parameter or it will try to redirect to your levels page.
There are a few ways to override this behavior by basically telling PMPro which level to assume on the checkout page if none is given. Here is one option, add this to your wp-config.php, active theme’s functions.php, or a custom plugin:
define("PMPRO_DEFAULT_LEVEL", "1");
Alternatively, you can set $_REQUEST['level'] in some code directly.
Hope this helps.