JK Rowling Fan Mail Help

[pinkomelette]

Hi, I'm planning on writing my 3rd fanmail letter to JK Rowling and sending it soon. The past 2 times I've written, I've attached an SASE but a different envelope has been used. With this letter, I don't know if there's any point in buying another stamp and sending an SASE with this one. Part of the issue is that I don't want to waste my mum's money and the other is that I've made a yellow envelope with this letter, and I don't want to stuff way too much in the envelope because I'm not sure if putting too much inside could maybe damage it? Do you think I should be ok stuffing in a normal envelope into it or do you think it's just a waste?