You can always automate your SMS service easily .
You can use any programming language of you choice. RingCentral offers APIs for both call and sms text messages and you can implement that automation through coding.
You can start reading API reference first to start with.
Check the call API docs here:
https://developers.ringcentral.com/api-reference/Call-Control/createCallOutCallSession.
And sending SMS API here
https://developers.ringcentral.com/api-reference/SMS/createSMSMessage
If you use Java, you can first call via API and than use SMS API including the invoice summary and implement it via Java SDK.
RingCentral already have different SDK and you can use those to implement the automation.
Ref https://developers.ringcentral.com/guide/sdks
Also regarding authentication everytime , yes Authorization is required always to send SMS using an API and the simple reason is to prevent misuse and abuse. It provides the security for an API while sending a SMS messages