That "posta" means "you are right, I agree, you are spot-on" or "seriously". If it was a response, then it's probably the former. It also means truth ('Decime la posta' would be 'Tell me the truth').
The "aguante" part is similar to "¡Viva!". It would basically mean "This is so good, hope it stays around a little longer", in this case. If you really love pizza you could say "Aguante/Viva/Dios bendiga la pizza".