I totally agree.  I selected Royal Mail large 2nd class to ship a CD.  Ebay had suggested a cheaper alternative though I knew that to be insufficient.  When the buyer paid and it was time for me to print postage, I was told the buyer chose Royal Mail 2nd class small parcel, and Ebay wanted another £1.55 from me.  I called Ebay and the person I spoke to basically said simple delivery doesn’t work well with CDs, and that I’d need to manually correct Ebay’s weight estimate of 10 kilos, and I couldn’t do that from the Ebay app!!! In the end, I paid for postage myself ((£2.00; Royal Mail 2nd class large letter) and posted it, so both the buyer and I have paid for postage. Ebay won’t refund me. I’d  suggest we all switch to another selling platform.