Can't change billing address

I'm trying to upgrade my phone on my prepaid plan by buying one through the website, but it won't let me buy one because it keeps using the old address I don't live at anymore as the billing address... when I have my current address on my account. Since the zip codes don't match with my debit card's zip code, it won't go through.

The option to change the billing address is greyed out on the checkout screen, and as far as I can tell in the account management menu there's no change billing address option. So, what gives? How can I buy this phone?

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