Make the box theirs.
Pick their favourites, or take the house mix if you would rather preserve the surprise for everyone involved.
Birthday cake has competition.
A box of cookies, picked for them, with your message inside. Candles remain your department.
Build their box
Add the useful bit.
Add a gift message in the box builder. Their address goes in at checkout.
Don't cut timing fine.
We recommend ordering with some wiggle room. Your courier doesn't know everyone's DOB. Hopefully.
Birthday sorted.
Well. The cookie part of it.