If you purchased a commercial subscription (and/or Telephony credits) inside your Bitrix24 account, you can find your invoice in PDF here: Subscription > Purchase History.
By default, Purchase History page is available only to the administrator. Other users will have access to the Purchase History page if the Enable all users to upgrade current Bitrix24 plan option is enabled in the portal settings.
If you purchased a commercial subscription from a Bitrix Partner (Bitrix Reseller), please request your invoice from your original Bitrix Partner.
If you purchased a commercial subscription and/or paid for a service inside a Bitrix24 MarketPlace app, please get in touch with the corresponding MarketPlace app developer.
If you are having any difficulties finding your invoice, or have any questions regarding your payments, please get in touch with sales@bitrix24.com (in your email, please make sure to specify your Bitrix24 account URL).