As a private seller I can see some benefits of Simple Delivery - it's generally good for buyers of items in the small/medium 0-2kg range as the rates are lower than standard RM shipping (which is what I use). The issue for me is that it does not account for the cost of bubble wrap, boxes etc.; if I didn't have plenty of boxes from my family's online purchasing habit, I'd be out of pocket. However, putting that aside, my biggest gripe is what to do with heavier items. Recently, I have sent out a few items in the 2kg-5kg range and Simple Postage requires I select "Up to 15kg" charging over £12 for what should be a £5/£6 charge. There is supposed to be a 2-10kg category but it does not appear. I also find many items are given a recommended 2kg postage cost BUT I know they weigh over that and am not going to risk falling foul of RM despite eBay's assurance that if they recommend that postage rate, that's all okay. I suspect I am going to ignore simple postage for most items and simply use my own - assuming it is not one of those categories where I am forced to use it.
One does have to wonder whether there is anyone who actually engages brain when coming up with this stuff. Why not allow the seller to add a packaging cost for example?