The main post office is below Piazzale Roma in front of the vaporetto stops. Note that you need to take a ticket on entering the building and then wait for your number to be shown.
You can buy stamps from post offices or tobacconists. The cost for a postcard stamp (francobollo) is 1 Euro for Europe, 2 Euros 20 cents to the US, 2 Euros 90 cents to Australia (correct July 2017). Full details of prices and weight limits and the relevant zones.
Be careful when buying stamps in a tobacconist in Venice
Some tobacconists in Venice do not sell stamps, they sell stickers for a private postal service - Friendpost. They are slightly more expensive than normal stamps (1 Euro 30 instead of 1 Euro for sending a postcard in Europe), they take much longer than normal post, and they can ONLY be posted in the post box in the shop. If you put them in a post box they will be thrown away!
Known as 'Venezia 5' on the post office website. It is close to the Strada Nova. Find the small campo where the Irish Pub is, facing away from Strada Nova turn left and then right into Calle Priulli. The post office is on the left just before the bridge.
This company can help with transporting goods around Venice or further afield. 24 hour shipping to most cities in Europe. 48 hours to the rest of the world. Will transport works of art, sculpture, glass in any dimension.