Are you in Mexico or Spain out of curiosity? Your post sounds like Mexico, but your profile says Spain:I recently received my RP, and, based on my experience, the above advice is correct. They took my RT when the request for RP was submitted. As Murphy would have it, a need arose for me to return stateside before receiving the RP, and I had to apply for a letter of approval to leave and return. It is a quite formal letter, and must be stamped at the border on your exit and your reentry. The explanation given me was that improper handling of this transit letter can negatively affect the RP application, so I would not recommend using one unless absolutely necessary.