They said they didn't know. Which is weird. The post office workers should know there's going to be an increase the day before it happens!
I only found out because I needed 3 more stamps (and then one lady tried to tell me postcard stamps were 22 cents, which I knew wasn't true, and the second lady told me they went up to 35 cents). If I had mailed 150 postcards and they all came back, that would have been a huge amount of money and time lost. Or what if they went to the publisher postage due?
The post office didn't have enough 1 cent stamps, so I ended up going to two different post offices, but happily, the postcards are gone and I don't have to worry about them any more!