It's going to run you at least $20, probably more.
I would recommend a shopping service instead. There are several good ones that charge a fee much less than a taxi would cost you.
Check out:
Gardengrocer.com
Wegoshop.com
Goodings.com
Basically, you contact them ahead of time with a list of what you want, they go purchase it at the store of your choice (except Goodings, that is at their store only), then deliver it. The fee will probably be less than a taxi, and the time saving well worth it!!