The eBay policy is very clear with regards to buyer requests for a change of postage cost/method.  Buyers can ask for a postage option that will cost more than what was stated in the listing, but policy says they can't demand a change to the postage method that was in the listing.

I would stick to the postage amount that was in the listing in the hope that the buyer decides to not pay.   If they do pay they have the opportunity to ding your DSRs.  I think if they leave a negative or neutral on the basis of a $7 postage cost rather than a $2 cost then that negative/neutral could have grounds for removal as the demand for $2 postage is against eBay's own policy, but removal would depend on them commenting about the postage cost in their comment.