Charity not found - trying to list an item and donate proceeds

[yogurt.weaver]

I have a small vintage book that will fit in a large envelope which I would like to sell and donate the proceeds to a local charity. I have already put the charity in my favourites, but when I go to "Donate a portion of this sale" on my listing, it can't find the charity! I have spent a couple of hours trawling through forums with no luck, I did change to "post only" from "post or collect" but that made no difference. 

Secondly, as it is a small book I should be able to opt out of "Simple Delivery" as that is charged at more than the price of the stamp I already have, but I can't do that either! 

Suggestions before I tear out the rest of my hair would be welcome.

[style_wise]

When you say you have the charity in your Favourites, do you mean they as a Seller on eBay? It would help to have the Charity name please.

SD should have Custom Postage available for all Large Letter under 100g items £10 or under, but it is glitchy and sometimes CP is not available. eBay are 'working on it' as of June.

[yogurt.weaver]

Thank you for your response.

Apparently adding your charity to favourites helps you buy from them! My mistake, found on the buying & selling to aid charity page. They do not sell on ebay.

The charity is Cotswold Canals Trust, registered number 269721, established in 1975,  which is listed as one I can add to favourites. But when I try to add the charity to my listing, I just get 

Search results

No charities found

The simple delivery is supposed to be optional for books, among other things.