Unable to upgrade phone

I have been trying to upgrade three of the phones on my account. The website only allowed one phone in the cart at a time, so I went through the checkout process with the first phone. After that, the website would not allow me to check out with the second phone; I kept getting different messages about not being able to use that credit card (I tried multiple cards, and none worked), and also a generic “oops, something went wrong, please contact customer support.” When I chatted with customer service, they told me that I can only upgrade one phone at a time, and I have to receive and activate the first phone before ordering the second one. This is frustratingly slow, but I waited a week for the first phone to be delivered and I activated it. Then I tried again to complete the checkout process with the second phone. I still got the same error messages. Two more customer support people told me that I should just keep trying, or go to a Cricket store— neither could explain what was wrong with the online checkout or how to fix it. The nearest Cricket store is an hour away. Why can’t I upgrade online?