Currently, adding a snapscan QR code to your invoices isn't an option in QuickBooks Online (QBO), @maryna-shaker-co. Let me provide further details about this. 

Based on your description of your concern above, it seems that you're using a payment application where you can pay through your phone. Since you're unable to add a snapscan qr code, you can manually type it in the Message on invoice field before sending the invoice to your clients as a workaround. Here's how:

1. Go to the + New button.
2. Select Invoice.
3. Enter all the necessary of the invoice.
4. Go to the Message on invoice section, then manually type in the code.
5. Click Save and close.

I've attached a screenshot below that shows the fourth step.

In QuickBooks, you create attractive and professional-looking invoices and add the information that matters most to your business. To learn more about it, you can check out this article for the complete details: Customise invoices in QuickBooks Online. The said article is also applicable for your quotes and sales receipts. 

Please let me know if you have other concerns. I'm just around to help. Take care always.