You will have to experiment until you find a category which allows you to use custom postage.
Sometimes, that may mean you list items in a category which is close to, rather than exactly, the category you would generally choose.
In jewellery, for example, if I list a pair of earrings or a brooch (Large Letter size and under £20) I can use custom postage.
If I try to list a brooch/earrings set, I would have to use SD so I would probably list my set as a brooch.
"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)