Payment Gateway
Function Tracker has a built in Payment Gateway that will allow your customers to pay invoices (in full) using Stripe or PayPal.
Initially you need to setup PayPal or Stripe via Side Menu > Admin > Third Party Integrations and also enable the option to show the payment method on your Payment Gateway.
Once you have connected to either PayPal or Stripe navigate to Side Menu > Admin > Financial Settings > Payment Gateway. Here you will find at the top of the page the URL of your payment gateway that you can provide to your customers, this can automatically be shown on your invoices also. Modify the Payment Gateway to suit your needs, then create a new test event with a minimal cost. Create an invoice for the test event and send it to yourself, then follow the link to make a payment via the Payment Gateway.
Once paid you should see from within Function Tracker the invoice has been marked as paid in full and also you will see a payment receipt with details of the PayPal or Stripe payment.