You can enable clients to pay invoices without logging into the client portal by sending them a direct payment link via email.

To enable this, please go to the Settings>App settings> Sales & Prospects> Invoices settings > General settings tab and enable the following option: 

After this, please add the {PUBLIC_PAY_INVOICE_URL} variable in the Send invoice email template. You can add the url inside a button also. 

Once you've set this up, every time you send an invoice email to clients, they'll receive a direct payment link, allowing them to pay the invoice without logging in. Clients can pay their invoices using any of the enabled payment methods.