Can I fulfill the Help Wanted task "collect 20 tips" by drinking the new Cafe "tips" drinks served..

You don't get tips from drinking the drinks other people have served. You get 20 tips in the cafe from serving a drink, and 20 tips each from everyone who drinks the drink you have served. Outside the cafe you get tips from some of the bushes gifted to you, but those don't count towards the help wanted tasks which specifically say collect 20 or 40 tips 'in the cafe'.

To satisfy the task, you have to put the coins into your jar (which adds to your tip total) after you have accepted those drinks. You cannot satisfy the task just by accepting the drinks.

I always leave tips uncollected after our coffee time. That way I can pop back quick and collect them to complete the Help Wanted task when that comes up.