求助，关于坐标轴问题

f[x_, y_] := 30 /; 0 <= x <= 500 && 0 <= y <= 5;
f[x_, y_] := 30 + 3*(y - 5) /; 0 <= x <= 500 && 5 < y;
f[x_, y_] := 55 /; 500 < x <= 1000 && 0 <= y <= 5;
f[x_, y_] := 55 + 4*(y - 5) /; 500 < x <= 1000 && 5 < y;
f[x_, y_] := 100 /; 1000 < x <= 1800 && 0 <= y <= 5;
f[x_, y_] := 100 + 5*(y - 5) /; 1000 < x <= 1800 && 5 < y;
f[x_, y_] := 350 /; 1800 < x <= 7000 && 0 <= y <= 15;
f[x_, y_] := 350 + 15*(y - 15) /; 1800 < x <= 7000 && 15 < y;
Plot3D[f[x, y], {x, 0, 7000}, {y, 0, 150},
axeslabel -> {货拉拉报价, 货物重量, 运输距离}]