As a Device Credit Agreement/Finance, there is a payment schedule within the document that would've been signed upon taking out the plan and you are issued with an Annual statement of the balance.
Similar to a loan, you are not necessarily invoiced for this as you agree to the schedule of payments on signing.
It could be worth contacting customer services to see if they can do anything to provide you with some kind of payment receipt, but your mileage may vary here (as you have discovered) as you do not officially get one.
