First, you must set up your Izettle and PayPal accounts as bank accounts in Manager. See the Guide: https://guides.manager.io/11367.
When you make a sale, enter a receipt. Select the correct account where the money is received. See this Guide: https://guides.manager.io/7820. Enter the items sold as line items and inventory will be reduced.
That is the basic approach. You also have the possibility of importing, for example, a PayPal statement as a bank statement. See this Guide: https://guides.manager.io/9240. Fees charged by these 3rd-party processors can be handled as bank fees. Search the forum on that subject, as different users take different approaches. If you are importing statements, the processor’s output format will determine whether you can set up bank rules to handle them automatically.