You pay for the RVIA sticker whether or not it is listed as an "option" because the manufacturer has to pay the RVIA and it is part of manufacturing cost.
If the manufacturer is listing the cost of the sticker in the build sheet or the price list, that's just an information choice made by the manufacturer, it is not likely that you can buy the RV without the sticker, because a RVIA member manufacturer usually does not have an option to leave off the certification. In some cases, RVIA members can choose not to certify export models.
But you could buy an RV from a non-member. I think a few of the smaller direct-sales manufacturers (LazyDaze maybe?) have resisted RVIA membership.
If the dealer has listed RVIA sticker as a dealer add-on, the dealer is just trying to get a few extra dollars.