How exactly do you calculate your shipping costs?
I suppose:
-) Membership only => free shipping
-) only real products => shipping calculated depending on what exactly???
-) Membership + real product(s) => same shipping cost as real products only (i.e. membership does not have any influence)
The first case can be handled by a condition "SKUs==list(1234)", where 1234 is the ID of you membership product(s). E.g.:
Name=No shipping for memberships; SKUs==list(1234); Shipping=0
The second case can be handled by a condition "not(1234 in SKUs)", i.e. none of the products in the cart has category 1234.
The exact rules depend on the specifics of you shipping cost structure.