The seller will look at this negotiation the same way as asking them to come down on the listing price 6%. It is subtracted from their net proceeds at closing and is not fees that they normally pay.
Some fees are also negotiable with the provider of the service.
Note that if it's worded only for closing costs, it cannot legally be used for the prepaids, points, principal or interest. In some cases if it's mis-worded and the amount that the seller is crediting the buyer is over the buyer's actual closing costs, the seller gets to keep the portion of the money that is over the closing costs.
You also have to talk to the lender FIRST to make sure that you know the max amount that the seller can credit the buyer. If you write a higher amount in the contract than the type of loan will allow the seller to pay, then again, the seller keeps the money.