I offered this as free postage (as I always do), and the ebay estimated shipping cost was deducted as soon as the buyer paid, and then immediately recieved an email with a postage label link. When I came to pack the only suitable box I could find was larger than the eBay estimate - in the past I was always able to pack, then weigh and measure and select best postage based on that. 

I'm usually going to have a good idea of weight/size but until it's packed I can't be 100% sure. This is what I was asking... how do I change Simple Delivery postage after ebay send me a label? 

As an occasional seller now trying to clear out a few bits post retirement, this new system is really putting me off selling on eBay!