Hey Baho and hello to the rest of the members,
I've been on this board for a while but this is my first post. I've been keeping piranhas for about 20 years so I think I can help you here. First Piraya are always wild caught and until about five years ago were very difficult to obtain.Anyway $90.00 for a 3" specimen is fair so go for it!
Forgive me if I rehash some stuff here.
One piece of advice I would give you is to make sure you get four or more fish. I currently have three 2" piraya which is normally a recipe for disaster but it's only temporary. I intend to pick up a couple more soon and put them in a larger tank.When you have three fish one is ALWAYS singled out and killed eventually.Having four or more fish tends to balance things out.
DO NOT feed your piraya beef heart as its primary food. Beef heart should be a treat for them every now and then. When you do feed them the beef heart make sure you trim the fat off. You can go to the market and pick up talapia fillet, cat fish nuggets, shrimp etc. If they take pellets great ,a varied diet is the important thing.
Piranhas are extremely nervous fish especially when small so floating plants and dim lighting are a great idea because it mellows them out.The other thing I would recommend is putting them in the largest tank you can. I believe the rule of thumb is 40 gallons per fish.
Good luck!