I don't know about Rio, but like settlers of cataan (mayfair games) They sell their replacement parts in packs, so you end up getting a lot more pieces instead of only what your missing.   Instead it's 4 color pack replacement for 12 bucks for the wood pieces.  For tiles, it's 27 bucks for the set.

So in my replacement shop, you could get 1 road if you wanted to or 1 water tile. something like that.  It would definitely be easier for game companies to do this but who knows, maybe they would consider outsourcing thier replacement services in the future.