"Gracias por
adelantado".
The use of "estar" is correct. That's where the data will be.
·Voy a estar en el restaurante.
·Estaremos en casa.
Edit: Your thread title has been corrected from "Can I say "estar en la un carta"?" to "Can I say estar en una carta?"
Either "la" or "una". Never "en la una carta".... and "carta" is a feminine.