...'how do I select appropriate postage AFTER a buyer has purchased if I offer free delivery?' 

I'm not sure there is any way to do this with a Simple Delivery label - apart from the 'cancelling & relisting' option, but you could decide to purchase your own postage outside of eBay, then claim a refund for the unused Simple Delivery label. There is a form to fill in, and you have to wait for the refund, so it is a bit of a fiddle - but it might be the easiest way out of your current predicament.