In your opinion is it legal to be Part 91 and deliver legal small packages and other expedited cargo? For example if I were to use my own small airplane, let's say a Cirrus SR-20, to fly expedited cargo from airport A to airport B for a fee. No alcohol, tobacco, firearms, or hazmat items. Mostly things like documents, aircraft parts, other things that are relatively small or relatively light weight.

I think you need a 135 operator certificate for that. legally at least.

That's what I was wondering. Too expensive to be 135.

Transporting persons or property from one point to another for hire, in which you provide the aircraft, Part 135 (or 121 or 125, depending on other factors). If someone else pays you to transport them and their gear in their aircraft, you can do it. If someone else provides the aircraft but accepts third party transportation for hire, then it's back to 135.

Your question is a direct reference to a 135 operation.

If you think the 135 is expensive, try doing it without the certificate.

Bird man, you're not the first and won't be the last person to think of putting their CPL ( or Private ) to use.
Trust me there'd be a bunch of other people already doing it if that were legal.
The answer is no.

The only way to do it is if you had a commercial and delivered packages/mail to for only one company and they owned the aircraft.

And there was no charge or profit from the delivery. And no advertising to carry for third parties.

There is a 135 option as the owner operator. A little research will turn it up. Best way would be to start with advisory circulars. Remember though that you would be required to do a 135 checkride with a representative from the friendly aviation administration.