Online Shopping Cart Does not work

I am trying to upgrade from a 3G phone to the promised no cost with traded in 5G capable phone.

Every time I go through the procedure, and the system shows I owe the sales tax, when I click on Check Out, a message appears that there are technical problems (actually, the amount due today is shown as zero) and to try again later.

This has been happening for more than a week (since my first attempt).