Have you not noticed Simple Delivery, which private sellers are obliged to use.
Custom postage can only be used for items in exempt categories up to 100 gms in weight and £20 in price.
The items you sell appear to all be in exempt categories so it should be possible to use Custom Postage.
I only sell in exempt categories with no problems!
Which item are you having problem with? I have tried several of your items List Similar and so long as price is under £20 Custom Postage is available.
