Why don't we just use calculated postage? Because it is not practical when buyers buy a few items together. It only works out when they buy one item.

It works for me. They send a request for a combined invoice, I send back an invoice with the correct postage for the multiple items. Admittedly that takes some manual effort.

It's not about the extra effort, for me it is about the ease for the buyer to pay. Most of my buyers pay straight away. I have automatic combining rules set up and that allows buyers to pay straight away without waiting for an invoice. I would never do anything to change that. My postage works out very close most of the time anyway. It's just that for some buyers it works out a bit less than the real cost and for other buyers it works out a bit more than the real cost. For me, I am even in the end.