Hi,
This may be 'old news' to some, but I found a work-around which currently allows me to choose my own postage option rather than having to use simple delivery.
When I list an item, usually as a 'buy it now' option in my case, I initially add a large value to the price box, usually £800 (other large figures may work?).
This seems to be the key to allowing me to add my own 'primary service', which offers the old style list of delivery services, which once chosen you can apply your own price to.
On occasion I have still had to choose the 'oversize' option for postage from their drop down menu, but once I clicked this option I could then choose a primary postage option from the same old style drop down list, for example Royal Mail special delivery from the express options available, I could also choose my own cost just as I could prior to the addition of simple delivery to the listing process.
It's then just a case of going back to the buy it now price box and changing it to what I actually want to sell the item for.
Sorry if I'm teaching any here to suck eggs, but thought it might help with some of the issues experienced above. I've used it recently to post out some expensive hubs that I wanted appropriately insured and a set of wheels that I wanted to send with Parcelforce rather than Evri or Yodel.
Matt