If you insert a merge field that involves payment in the Folio, the field will appear blank if there aren't any transactions in the Folio yet.
A common request from customers is for Charges, Rents, and Payments to be seen on a confirmation. If your business policy is to place the RENT charges on the folio at time of booking, then 'Display All Folio Items' will be the right choice.
However, if you pre-book some Charges, such as a spa or transportation charge, but let Night Audit post the RENT charges (most common), and you wish for a Guest to get a clear indication of what has been charged and will be charged, then use the 'Display All Folio Items' for the Charges posted and, in another section of your Letter, the 'Daily Rate Grid' so that 'current' charges on a folio are on one portion of the letter, and the 'projected' Rate Grid is on another portion of the letter. This allows the Guest to see what will be charged, assuming dates and rates don't change.