Hi there and welcome to the Forum!
At the point of top-up, an invoice for the top-up amount is created, which you can access and download in your billing history (see Settings > Organization > Billing > Billing History). You also receive an email confirming the top-up, which includes the link to the billing history.
Also, I am not sure why you would still receive invoices at the end of the month given you have been migrated to pre-paid billing 
Anyway, let me know in case anything is still unclear.