So, I decided to take the plunge, and cut-back on my monthly plan from a $40 to $15 come April 2020. As mentioned by @darlicious I should take the plunge before they change the prices on me. Better early than late in this instance!
Scroll down the page, under your Overview > Account Status > My Data & Add-Ons. You will see what I have now. Click on purchase add-ons. This will avoid any confusion if you click on the Data that is 4G LTE. The data add-on for the $15 reduced rate (special discount) will be at 3G speeds
After clicking on the Purchase Add-Ons, pick your add-ons. If you can't see what the small print is, increase your viewing web browser Command + (mac) or Control + (PC).
Make sure this is what you want. Click on Buy-Add-Ons. Don't worry, you will still get another chance to confirm your buying add-ons.
Another screen will pop right onto the add-ons:
If you are good to go, click on Confirm my purchase. If not, go back. Don't want it, close the window! Because I want to purchase this add-on just because I want too and not because of this step-by-step tutorial because I have too!
It will take a minute or two. In my case, it took 30 seconds! Just be patient.
If you are lucky enough to confirm your purchase, you will notice my available funds at the beginning was $51. Now it's $31. Meaning, my $20 add-on was a success!
Are you still not 100% if your add-on was actually added on to my account. No sweat, here you go. Click on return to overview, look at the Overview > Account Status > My Data & Add-Ons. You will see what I have now.
Congratulations, you have just added the add-ons.
I hope this tutorial was helpful. Have no fear, you can do it.
@will13am and @gblackma, I have modified the title to be How to Buy an Add-On (Public Mobile 2020 Interface) so, that people know this is the recent look of the interface.
It seems people get a bit confused when they see a different interface. It's like using Adobe Creative Cloud apps and people say, but, this doesn't look the interface of Creative Suite 6! It's the same processes I feel, just the cosmetic of it.
I did it because I was sharing my experiences in the process when I did buy the add-ons. No disrespect to @srlawren who did an awesome job in 2017, and no disrespect to anyone else either.