Make sure you have selected to use Royal Mail only. You can still arrange for Simple Delivery items to be collected by RM.
When you list your item, you need to choose medium parcel - which is for items up to 61cm. That option is available under 'package size'.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)