I ordered a USBtiny a few weeks ago from ebay to use to program the garden station and it arrived today!
Got it up and running an it works great! I was previously using a spare Arduino Uno board but this new programmer is simpler and doesn’t require a bunch of jumper wires via a bread board (also frees up the Arduino for something else). The power also seems to work to power the board via the programmer.
I had to bend back the spare data pins on the prototype board (they were right next to the ISP pins), however I’ve already moved them on the SMD version of the board.