What happened in the delivery guy sketch was, the fella brought out one of the packages and, in an irked tone, asked Herbert to sign the slip. Herbert steadfastly responded:
"Not until we make the sound!" with an onomotopoiea rhythm.
The delivery guy responded:
"blah-ble-blah-ble-blah-ble-bloun"(???) with that same rhythm.
You know how some kids (and a few non-kids at that) make fun of others by repeated what they just said with a fake lisp? That's what the delivery guy was doing. Some folks do that when there's no "legitimate" point to ridicule them on.
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As for the sketch with Grover holding up the heavy blocks reading "HELP", after Herbert Birdsfoot had placed two extra HELPs on Grover's load, HB responds to the Help! cry with: "You need MORE help?" Grover replies that he does not; everything is just fine. HB leaves, and Grover then collapses backwards under the weight of the three blocks. Now off-camera, we hear Grover let out one last, subdued but desperate, "Help!"