Is there a way to get the invoice internal ID into a custom extension's service controller? I can see the model.internalid is available on the TPL in the theme, it's not clear how I can reuse this value in a custom extension though.

You would do something like

container.getLayout().getCurrentView().model

I'll try, thanks.

Yeah at that point in the extension timeline, there is no layout

Probably needs to be inside here then

layout.addChildView('Invoice.PaymentLink', function () {

You could pass the application object to that

Trying this now (just takes a while to activate)

mountToApp: function mountToApp(container) {
    var layout = container.getComponent('Layout');
    console.log('PaymentLinkOverride: layout', layout);

    layout.addChildView('Invoice.PaymentLink', function () {
       console.log('this', this);
       var model = this && this.model;
       console.log('model', model);

Well you should probably pass

container

I did this

return {
    mountToApp: function mountToApp(container) {
       var layout = container.getComponent('Layout');
       console.log('PaymentLinkOverride: layout', layout);

       layout.addChildView('Invoice.PaymentLink', function () {
          return new LinkView({
             container: container
          });
       });
    }
}