What happens to extra toner if a full page is printed to a half-sheet of paper?

Ideally, the majority of the unused toner is recycled back into the cartridge. Most cartridges, however have a page-counter that will tell you the cartridge is low/empty after X pages... rather than have any kind of sensor telling if it's actually empty/full. So.... if a cartridge says it'l print 1000 copies... 1000-blank pages = assumed empty. Sometimes they have some heuristics that try & guess the amount of page-coverage and do a better job of estimating the actual full-empty status...

Alternatively... you frequently can do page-scaling on HP printers and set it to fit 2-per-page instead of printing on 1/2 of a page and cutting it in half/throwing the half away...

You should be able to specify a user supplied page size in the printing dialogue.

Just set it to 8 1/2 by 5 1/2 and you won't have to worry about extra toner.