γ = {40, 0}; μ = {4/3, 60/4};
r = {{0, 1}, {0, 0}};
c = {∞, s};
shoppingProcess[s_] := QueueingNetworkProcess[γ, r, μ, c];

Manipulate[
 QueueProperties[{shoppingProcess[s_], 1}, "MeanSystemSize"] +
   QueueProperties[{shoppingProcess[s_], 2}, "MeanSystemSize"] //
  N, {s, 3, 5, 1}]