You got some ... if I may say so ... considerations.
1) You need a reason this delivery guy has 'magic mushrooms' handy.
2) The delivery person has to know the restaurant person would
eat the stuff,
would eat enough of it to get sick,
would eat it within a certain time frame,
and would not share it with anyone else.
(One woman gets sick ... she goes home and the restaurant brings in somebody else for that evening.
Two people eat the evening's offerings and get violently sick -- they stop serving food RIGHT THEN and close the place.)
3) You got a scenario where the woman gets really ill from just tasting the food.
What happens if somebody has a whole bowl of soup?
Yerk. Slerk.
(drops dead.)
This stuff is not without a certain toxicity.
4) Before he hauls out the mushrooms,
you got to convince the reader there's some good reason
the delivery man doesn't just,
sneak a dose of ipecac or visine into her coffee,
or rub poison ivy all over her clothes the night before,
or break into her house that afternoon and nip holes in her pipes and call to say the basement is flooding
which all seems easier than mushrooms which are likely to be traced back to the 'Eat this nice muchroom, girlie' deliver man.