The on-line Venice Connected allows you to pre-book transport passes, tickets for civic museums and public toilets with variable discounts. You must book at least one week before you arrive; any later, you pay full price.
Venice Connected is a multi-service system that can save you money on travel and museums in Venice if you buy tickets before you go. When you buy online you get a PIN code you can use at all the amenities you buy into – a very flexible way of buying discount tickets, but you have to decide what you want to do before you arrive in Venice. You can add all or almost nothing to your online shopping basket: return journey for the airport by waterbus, weekly waterbus pass, entry to all the civic museums (you can even get a cheaper price if you choose off-peak visits only, eg afternoons), entry to all the public toilets dotted around the city; even wifi access.
Because the prices vary according to season and the predicted flow of visitors into Venice, it is hard to quantify the savings in any headline way, but essentially if you do what you plan to do, you will save! As an indicative figure: return airport transfer + seven day waterbus ticket + toilets = €68.