Is it me or is there no way to add any extra items to an already existing subscription delivery?! Seems a bit mad to pay a load of postage when there’s a shipment coming out anyway 
You can do it but not for just one order. You can add products to your subscription, you then just need to remove them once the subscription has been processed. Some products can’t be added like the accessories because generally people don’t want these to arrive every order and unless you manually remove them they have mistakenly arrived with every order in the past.
Hope that helps!