Del Diccionario Jurídico de Ediciones Heliasta:
carta de crédito = letter of credit; bearer draft.
Intento tu frase...
Contrata una empresa certificadora al pago de la carta de credito ya que tus medidas (sizes) son exclusivas = since yours are exclusive sizes/since we're dealing with exclusive sizes, (it's best to) hire a certifying company upon payment of the letter of credit.
Saludos,
LN