Sending invoices via sms messages can be done manually, blessedandpresse.
I'll be sure to guide you through the process so you can begin sending SMS messages, including invoices, through QuickBooks Online (QBO).
You can achieve this by sharing the link to your invoices, manually copying it, and then sending the invoice link through SMS outside of QBO.
Here are the steps:
On the invoice page, select the drop-down arrow next to Review and send.
Click the Share link.
Hit the Copy link button to send the invoice link through SMS outside of QBO.
If you want an automated solution, I recommend looking for a third-party invoice management application that can send SMS messages with invoices included automatically.