I'm here to share with your some insights about processing invoice payments online in QuickBooks Self-Employed (QBSE).
The PayPal Beta integration for invoice payments has been discontinued already. Thus, clients can only process payments using cards or bank transfers. However, you can connect PayPal as a bank through the Banking page.
Here's how:
Go to the Settings or Gearicon, and then select Bank accounts.
Search for PayPal. If you've already connected an account before, select Connect another.
Select it from the suggestion, then hit Continue to PayPal. This will route you to their website.