Why does everything cost more when I sit down?

Because Italian bars have two price lists: standing at the counter (al banco) and seated at a table (al tavolo), and the difference is legal and posted. An espresso is €1.20 standing and can be €3.50 or more sitting, and in a famous piazza far more. You're paying for the seat and the view, not being cheated. Do what Italians do: drink coffee standing at the bar in thirty seconds, and save the table for when sitting down is the point. This is the single biggest daily saving available in Italy, and it also happens to be the better experience.