Hi! For the anon who asked, you can actually buy them in mobile. You put the amount you want to pay and then your card info. You then get an email from gumroad that can take you to where you can download the collection. Now on iPhones from here you can open it into iBooks but I’m not sure about androids or other phones. But like I said I’d bought all four of the collections on mobile and I haven’t had any trouble.