W3 Total Cache and PayPal paying customers with s2member PRO

[dtelepathy]

We're having a problem with s2member and W3 Total Cache when customers attempt to pay with their PayPal account offsite. When a customer opts to pay with this method, when they click the final "Continue" button in the PayPal checkout interface to submit their payment, instead of the payment being processed, they are immediately redirected back to the checkout page with the s2member_paypal_xco _GET value set to s2member_pro_paypal_sp_checkout_return, and the token and PayerID _GET values set as well. Normally, the customer would be redirected to the appropriate, specified thank you page and the IPN processed. I have watched the IPN logs and nothing goes through, nor does the actual transaction in the PayPal interface when W3 total cache is turned on.

If I am logged in to the WordPress system and run through the checkout procedure, everything works fine as we have W3 total cache set to cache nothing for logged in users. In the template files for the checkout and thank you pages we have the W3 total cache constants defined to prevent page, db, and object caching set:

define('DONOTCACHEPAGE', true);
define('DONOTCACHEDB', true);
define('DONOTCACHCEOBJECT', true);

We also have the checkout page and thank you page all listed as pages not to be cached in the admin interface for W3 total cache as well as the option to cache pages with _GET parameters defined turned off.

Is there another setting that I am missing to get W3 total cache working with s2member? I would really like to use this caching plugin due to its capability for tight Amazon S3 integration, verbose caching configuration options, and ability to integrate with caching systems such as APC and Memcached. I'm using Quick Cache in the mean time until we can get this resolved, but it doesn't have all the features I'd like to keep our site speedy.