The first payment for your subscription is charged immediately when you place your order. This ensures that a valid payment card has been registered and can be charged before your device is shipped.
After that, recurring monthly payments are charged three days after your order date. For example, if you ordered your Flex subscription on the 15th, future monthly payments will be charged on the 18th of each month.
The three-day delay accounts for the delivery time before the subscription begins to be used.