Disable IPN per specific transaction

Hi!

I have IPN enabled in Paypal according to s2member instructions and everything regarding s2member works great.
I have a problem with paypal payments that are not s2member related. With these payments Paypal still wants to notify my site (s2member) that transaction has occurred but then an error is generated and shown to the customer.
So, I would like to be able to disable IPN notifications for these transactions. As far as I understand I should do this in Advanced variables in the Paypal button generator. How could I do this? What should I put into the advanced variables box?
Thank you.

Anyone? I am quite desperate :(

Does the bellow excerpt mean, that I could change the Paypal IPN setting from the default one (http://www.mydomain.com/?s2member_paypal_notify=1) and everything would still work?
I think the reason why customers see the error after checking out with paypal after purchasing non-s2member related item (ERROR: Unable to verify $_SERVER[“HTTP_HOST”]. Please contact Support for assistance.) is because Paypal is trying to notify my site of the money transaction that s2member does not recognize. Would this make sense? How can I “disable” IPN notifications for all payments except s2member related purchases?

*Quick Tip* In addition to the default IPN settings inside your PayPal® account, the IPN URL is also set on a per-transaction basis by the special PayPal® Button Code that s2Member provides you with. In other words, if you have multiple sites operating on one PayPal® account, that’s OK. s2Member dynamically sets the IPN URL for each transaction. The result is that the IPN URL configured from within your PayPal® account, becomes the default, which is then overwritten on a per-transaction basis. In fact, PayPal® recently updated their system to support IPN URL preservation. One PayPal® account can handle multiple sites, all using different IPN URLs.