Like ambersun mentioned here, Amex charge cards need to be paid in full every month. There are no interest charges, but there are fees for late payment and possibly account termination. Each different kind of charge card also has an annual fee, and the Zync has the lowest one of $25 per year (though the reward points are worth less than those of the Gold card, etc.). I suppose this would help people develop good habits because it almost forces people to always pay in full. Charge cards are reported to credit bureaus and show up on the report, while debit cards are not reported. I’ve heard of some people using them to build credit. I’m still not sure what exactly they report to the bureaus, as I’ve read some people say the highest balance is reported and some say other things are reported. This reporting issue is complicated because Amex charge cards have “no (stated upfront) pre-set spending limit”, so it does not have a “credit line”.
I heard that GEMB is very generous with credit limit increases, so they are good store cards to get. I was looking into the PayPal Extras MasterCard that’s issued by GEMB, since I use PayPal a lot and already have a verified account with them. However, there seems to be many problems accessing the account from the PayPal page, so I do not think I will touch it. I may look into the GEMB cards you mentioned too.