I have done some research/testing and found these results:
It seems that Paypal has recently updated their Paypal here app and is only allowing it on devices they have tested themselves. I have two other devices that "have the requirements" but are not "compatible". With the popularity of the commando vs, for example, getting it on the galaxy note...
So then i tried to forcibly install it. they are on version 1.7.2. the most recent version that I could find for a sideloading was 1.5.x. It installed and ran fine, but i only got to the login screen. (not 18...yet) finding if it all works would be up to you. The most i can do is post the apk here. (see below)
Essentially, you would be at the mercy of Paypal for allowing slightly behind versions to work, and at the mercy of xda developers for people to post the latest apk. i know that in a business world this reliability would be considered unacceptable.
As for an update on the phone, I have heard editing of the build.prop file, but that might make any Casio system app stop working. again not reliable.
You can download and try the version i posted, and i wish you best of luck, but it will only work for so long until Paypal updates again...
Another thing you should note is the use of a audio jack. the commando one sits lower than other phones, so you might have to buy a physical workaround. I went to amazon and got a "mediabridge" cable and a female to female adapter. there shouldn't be a problem with the card reader beyond that.
If you want to buy a phone to work with Paypal I would just call them up and ask them what android phones work with it right now. then ask them if the commando 4g works
I wish i could help out further, but i dont have the account or paypal card reader to test with. Anyway, hope this helps and best of luck