$20 -$40 or $50, depending on how often they come around and how good their service is.
Place the cash (no gift certificates) in a "Thank you" card and write "Thank you for a professional and courteous service." Give it to them on the SECOND to last day before Christmas, as to not miss them if they don't show for some reason on the scheduled day before.
While handing it to them, smile lovingly and do not take it back if they reject the gift. Insist on it for the health of their family and illegitimate children. God bless us all.