Shadchanim are halachicly entitled to collect a fee. IOW you are halachicly obligated to pay the shadchan a fair market fee for his services (unless he agrees to waive it.) See Rama C.M. 87:39 and 185:10. Doesn’t matter if he is a part-time, full-time, volunteer, etc. shadchan. (Beiur HaGra, ibid. and Teshuvos M’harash Engel 3:15.)
Also see Ketzos HaShulchan 107:8; Kaf ha-Chayim 306:50; Erech Shai E.H. 50; Reb Akiva Eiger C.M. 185.