Title text: I'm on the USPS no fly list
We see Cueball on a phone with someone who sends various strange things through postal mail. The third panel is a pun on the usage of the phrase 'a lot of time.' Normally, this means that something will take a while to finish; in this case, however, it means it is literally a large quantity of time that is being sent.
Quantizing time ("discrete packets of time") is a theory that time is not continuous as particles in the quantum mechanics. It could be one of the big mistakes in modern science, but feels as if there's more to it, in the world of romance, sarcasm, math, and language.
The title text states that due to his habit of mailing strange things, he has been barred from sending mail through the United States Postal Service (USPS).
A no fly list is a list of people who are not allowed to use commercial airlines for travel in the United States. It is maintained by the Terrorist Screening Center. Here, the person sending strange objects through mail is on the USPS no-fly-list for people. The joke here is that USPS does not have such a list for people. There is a list of items banned from being shipped by USPS, which includes most consumer electronics with lithium batteries. It could also imply that the sender attempted, at one point, to mail themselves via air mail.
This comic might be related to W. Reginald Bray, an Englishman from the turn of the 20th century, who was famous for mailing unusual objects (including himself) to experiment with the postal system (detailed link here).
A list of the things:
A running chainsaw: While a chainsaw may be mailed with risks and precautions, a running chainsaw would be very dangerous to mail assuming there was a constant power source.
A baby's first word: This is a sound, and thus can not be mailed. It can be recorded and the recording mailed.
A blank stare: This is abstract, and the closest to mailing it would be a picture.
A dizzying height: Like the above, this is abstract.
Pi: A mathematical term close to 3.141592653589793238462643383 but is infinite and thus can not be mailed.
Time: This is completely abstract and cannot be mailed. 
- Phone: Do you think I could mail a running chainsaw to someone?
- Cueball: I doubt it.
- Phone: What about a baby's first word?
- Cueball: Look, your obsession with sending strange things through the mail is getting out of hand.
- Phone: Can you mail a blank stare?
- Phone: A dizzying height?
- Phone: Pi?
- Cueball: ...
- Phone: Well, did you at least get that package of time I sent you?
- Cueball: I... you... no, I didn't.
- Phone: Well, there was a lot of it, so it will probably take a while.
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