ecommerce payment methods

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I have a client that offers both a website subscription and physical products. Physical products can be invoiced or paid via Paypal. How can we setup the checkout so offline Invoice is NOT an option customers can select when becoming subscribers.
I was thinking of creating a product attribute for the membership product and getting the offline invoice (default) gateway to check if any product in cart contains a membership attribute? Just not too sure on how to implement.
Membership is controlled by the Moortown I Belong addon. Works well, just don't want subscribers to be able to select offline invoice as their payment method.
Any pointers in the right direction would be much appreciated.

Thanks