A friendly advice is to check the credit card payment settings...
I used the download -> USB -> Car option when purchasing an update. One thing that I found out was that if using credit card payment and the purchase is made from Mercedes in Germany, the payment goes to Germany (of course). This requires the credit card used to be set to allow for payments to countries outside of the country of residence if this is not Germany. My bank offered blocking such payments in order to prevent fraud. When first attempting to pay, the Mercedes home page returned a failed payment message but there was no indication as to why. Once the blocking was unset, the payment was made and the update could be purchased