The local newspaper announced yesterday some possible (probable? already confirmed?) changes in the Tourist Travel Cards, also known as the "vaporetto passes".
ilgazzettino.it/NORDEST/…816084.shtml outlines some of the changes (in Italian).
The 12h and 36h passes are eliminated. The 72h (3day) pass will cost €40, and the 7day pass will become €60. Also the VenziaUnica card will become €50 for those who reside outside the Veneto region.
Currently the Veneziaunica website seems to have the products still on-sale at the old prices, so it may be a chance to get a last minute discount afterall, though I wouldn't wouldn't put it beyond the ACTV to require the extra cost at the pickup of the tickets/passes...