You can add a payment link to your invoices. Your users can then click the said link to pay for the items on the invoice.

Adding a Payment Link to Invoices

Use the following code snippet to add a payment link below the items on your invoices:

add_action( 'getpaid_email_cart_totals_total', function ( $invoice ) {
	/* @var WPInv_Invoice $invoice */
	if ( $invoice->needs_payment() ) {
		printf( '<div style="margin-top: 10px;"><a href="%s">%s</a></div>', esc_url( $invoice->get_checkout_payment_url() ), __( 'Pay Securely Online' ) );
	}
} );

The above code snippet first checks if an invoice requires payment. If yes, it displays a payment link below the invoice total on the invoices list table.