If you are requesting money using a PayPal link, you might be interested in the PayPal Payments plugin.
After you have installed the Plugin in Overview/Plugins/Get Plugins, you will get a new item called “PayPal Payments” in the Plugins section.
This will lead you to the PayPal login. Once logged in on your PayPal account will show up directly in GrandTotal. In contrary to the appearance in a Browser you will notice an “Assign” button to the right of your PayPayments.
Clicking this popup menu will check if you have one or more open invoices that match the amount and you just can assign the payment to that invoice.
Just in case you are wondering. There is nothing evil about this plugin. It just patches the visual appearance and adds this button.