We're staying on the Lido in Venice for 2 nights, then the next 2 nights in the San Paolo area. We've purchased and online tour of the islands (Murano, Burano and Torcello) so we won't need to buy transportation there. If we think we'll save money buying the 72 hour travel pass, is it cheaper to buy it online? Also, I've seen some posts about buying a pass that includes some museums but I can't find any information about that.
You used to be able to to save some significant money buying the passes on line but as far as I know that has pretty well disappeared. It is very easy to buy a pass on arrival at the train station or Marco Polo airport, or in P Roma if arriving by car. The passes cover all the vaporettos in the lagoon except for the 17 car ferry, and if course don't cover the Alilaguna boats or the private taxis. That does include both Lido (and north of there at Punta Sabbioni) as well as San Michele - the cemetery - and Burano, Torcello and Murano. A single trip will cost €7 at last check, divide 7 into the price of the pass and you will know how many segments will break even. We never combine museums with transport in Venice, but some others may.
You can pick up a separate Venice card from the HelloVenezia booths.
http://www.hellovenezia.com/index.php?option=com_content&view=article&id=173&Itemid=153&lang=en