It; depends where you are getting it from, they'll normally state if they deliver to your door. Most places who let you buy there fish online will also deliver. I've ordered from tropicalfish.org.uk before and they arrived around 10am the next day .
If you go to tropical fish finder and use the shop wildwoods they deliver to your door and have various species of piranha other than just the common nattereri.
Also try piranha warehouse, Dave can do you a good deal