Not really something we can answer as they might have altered the method again (though I really hope they haven't!). They did cobble together a quick discard option via your inventory last year; once the howling of dismay over how slow their cutely animated method was got so loud that it could be heard from space.
Ofc they didn't tell anyone they'd done this, they just left it for you to discover it, and some people didn't, for days, but that's beside the point. That enabled you to donate up to 15 items at a time. You did need to check each box, there wasn't a check all button like in Quick Stock, but if they use that method again this year hopefully there will be! It was quick and easy once this method was there though.