There seems to be no trace of any such custom on the part of almsgivers, so that the expression must be taken as a figurative one for making a display. It is just possible that the figure may have been suggested by the “trumpets” of the temple treasury - thirteen trumpet-shaped chests to receive the contributions of worshippers. (See Luk 21:2.)
Have their reward (ἀπέχουσιν)
The preposition ἀπὸ indicates receipt in full. Rev. renders they have received, so that there is nothing more to receive. So Wyc., They have received their meed.