Since Rigid Letters are more expensive, can I use Flexible instead?

Size can be a problem here, back height between Flex and Rigid is not always the same. If the same size is available, a flexible letter will fit into a rigid track (a rigid letter is too thick to go into a flexible track). A Flex letter is “gripped” top and bottom when inserted into the proper track, but is often very loose when used in a track meant for Rigid letters. Because they are thin, a flex letter can fall down behind the rigid track and not stay in place. And when not used with the proper track, they tend to blow out in the wind, or to be blown together and bunched up. Another factor to consider is that flex letters WILL NOT work with a change pole. For these reasons it is not recommended.