Back from Yogyakarta, I was only there a few days so this is a small update:
This shop still exists, and still has postcards. I could see that from the outside but I was not able to go back and shop there during opening hours, so I don’t know the price. The opening hours are like 11am to 5pm or sometimes 12 to 5pm. Mainly on Fri-Sat-Sun but maybe other days - the owner seems to update via Instagram daily. But because I was always elsewhere sightseeing, I wasn’t able to go back.
I found postcards tucked in in a tourist shop in front of Mall Malioboro. It was not a bookshop, it had souvenirs like many others, and an ice cream counter. But we went inside and all at the back, not visibile from the street, was a postcard rack. The price was IDR 13,000. That is a lot more than the prices listed in this thread from only 3-4 years ago! The postcards were good quality and seemed similar to those sold at the Lucky Boomerang bookshop - I would guess the price in the bookshop is similar nowadays.
I didn’t see any postcards for sale at the main post office.