I had the same problem with a voucher when i placed my last order, i gave up in the end and lost my £10 discount as i didn't want to loose my item and CS was closed.
I saw the one to own this morning and noticed they were NOT screw backs so i was about to post and tell you but when i went to add the link i noticed the screw post and thought i better not post in case i had got it wrong but i was sure they were post and butterfly, hope you get it sorted.