Tax Invoices will be emailed out for each billing period. If you cannot find yours, you can request a duplicate from billing @coviu.com.
These will be sent to the email you have nominated under Billing details in the Billing + Plan section of your Coviu account. Details for how to change your billing details can be found here.
You can also find a record of the payments you have made in the Billing + Plan section of your Coviu account under Invoices.