TfL has turned on the ability to collect online orders (top-ups and travelcards) on buses. At the moment it is in a test phase which means that it may not work if an issue is encountered.
Testing began on 1st September but had to be halted a few days later as there were some significant issues. It was restarted on 2nd October and so far there have been no problems. I’m aware of several people who have picked up top-ups on buses in the last week, including my son. If all goes well it is hoped to launch it properly at the end of this year.