Any tips, tricks, or good advice for lowering the cost of the fold 3?

There are no " tips or tricks" . There is a valid trade-in offer ($800 off) which you can read on the sales page for the Fold.

https://www.att.com/buy/phones/samsung-galaxy-z-fold3-5g-256gb-phantom-black.html

You must be trading in a fully paid off phone. If you are still under installments for the s20 you are not eligible. 

Make sure you expand all of the arrows and read all of the fine print involved in the trade-in process.   Doing the trade-in at a physical store is the best way to get a receipt proving that you traded in your s20.   Read the receipt. It  showed show the trade in the code and the phone you traded in.

Take photos, document everything.

Make sure you are on the unlimited data plan at the time of purchase. If you are not on on the limited data plan you have to change plans or you will not receive any credits. Credits are over 36 months, again read the fine print, if you upgrade early you stop getting credits.

Thanks for bringing up those points.

I was under the impression that it was wiser to upgrade while the phone was still fairly new so I will be checking into that and I do appreciate you bringing those details to my attention.

@CherryBomb

Noooo.

This isn't like trading in old car for a new car.  It's not a book value trade (and cell phone "book" is incredibly low)

You're going to get the same $800 trade-in offer for an S20 as you would for an S9. And in both cases you have to trade in a phone that's 100% paid off, and since the s20 is not even two years old there's a good chance you're still owe some money on it.

Not a good deal for you. 

In fact none of these deals are designed to save you money oh, they are designed so that the service provider can keep you a long time and make profit off of selling you high-priced service.