The Venetian government issued prepaid letter sheets— the first offically sold prepaid postal stationery.
At the top of the sheets the letters "AQ" (a contraction of acque) were printed, as the pre-paid sheets were intended to generate revenue for the repair and upkeep of the waterworks in the city by the Collegio alle Acque. Below the large letters "AQ" and the lion of Venice was a statement of the statute by which the system operated with a surcharge of 4 soldi on the cost of posting a letter. Each sheet had an identification number printed at the top left, and the name of the revenue officer by whom they were issued. The system remained in operation until the end of 1797. (Samuel Gedge Ltd., Rare Books Catalogue V  97.)
A value of these sheets was that the user could assume that the letter would definitely be delivered. Most private postal services operating at the time charged the recipient for the delivery with the result that mail was often refused.