A red sticky label which can be attached to anything in the game, or removed again.

"Brown"
The Shipping Room is a room. The red sticky label is a thing carried by the player. The description of the red sticky label is "It reads: AIRMAIL[if the label is part of something (called the parent)]. It is stuck to [the parent][end if]."
A black crate is in Shipping. The description is "A boring black crate." The brown crate is a thing in Shipping. The description is "An ordinary brown crate."
After examining something when the label is part of the noun:
   say "A bright red sticky label is attached to [the noun]!"

Here is the essential point: whenever we ATTACH LABEL TO something, it becomes part of that object.

Instead of tying the red sticky label to something:
   now the red sticky label is part of the second noun;
   say "You stick [the label] to [the second noun]."

And of course the label cannot be stuck to itself or to more than one thing at a time.

Before tying the label to something when the label is part of something:
   if the label is part of the second noun:
      say "[The label] is already stuck to [the second noun]." instead;
   otherwise:
      say "(first freeing the label)[line break]";
      silently try taking the label;
      if the label is part of something, stop the action.
Instead of tying the red sticky label to the label:
   say "That would ruin the label entirely."
Instead of taking the label when the label is part of something:
   now the player carries the label;
   say "You peel the label off again."

Much of the rest is just tidying to make sure that the player's commands are redirected into the right syntax.

Instead of tying something to the label:
   try tying the label to the noun.
Instead of putting the label on something:
   try tying the label to the second noun.
Instead of inserting the label into something:
   try tying the label to the second noun.
Understand the commands "stick" or "apply" as "tie".

We could have created a new "sticking" action, but to keep the example short we will use the built-in "tying" action instead, and respond to the command "stick" just as if it were "tie".

Understand "peel [something]" or "peel off [something]" as taking.
Test me with "i / put label on the black crate / look / x black / x label / get the label / apply label to brown crate / look / x brown / peel off label / stick label to label".
Test me with "i / put label on the black crate / look / x black / x label / get the label / apply label to brown crate / look / x brown / peel off label / stick label to label".
Shipping Room
You can see a black crate and a brown crate here.

>(Testing.)

>[1] i
You are carrying:
a red sticky label

>[2] put label on the black crate
You stick the red sticky label to the black crate.

>[3] look
Shipping Room
You can see a black crate and a brown crate here.

>[4] x black
A boring black crate.

A bright red sticky label is attached to the black crate!

>[5] x label
It reads: AIRMAIL. It is stuck to the black crate.

>[6] get the label
You peel the label off again.

>[7] apply label to brown crate
You stick the red sticky label to the brown crate.

>[8] look
Shipping Room
You can see a black crate and a brown crate here.

>[9] x brown
An ordinary brown crate.

A bright red sticky label is attached to the brown crate!

>[10] peel off label
You peel the label off again.

>[11] stick label to label
That would ruin the label entirely.