Shopkeepers bill you for using their bear trap or land mine

src/apply.c

@@ -2880,6 +2880,7 @@ use_trap(struct obj *otmp)
         }
     }
     You("begin setting %s%s.", shk_your(buf, otmp), trapname(ttyp, FALSE));
+    use_unpaid_trapobj(otmp, u.ux, u.uy);
     set_occupation(set_trap, occutext, 0);
     return;
 }