From what I can see, I sense they've cut out the middleman that handled the postage side for them and Simple Delivery is eBay's in-house solution to replace it. This would explain why they can't translate the framework from the 'Get Postage Label' to the new Simple Delivery, I think the middleman here is Packlink and they have an integrated solution for Collection that eBay haven't yet been able to figure out.

To re-use this code would involve keeping the middleman in on the cut so it seems they've opted to rework delivery options from the ground up for maximum profit, I wouldn't be surprised if Personal sellers are the guinea pigs here and they will roll it out to Business when they've tweaked it (response so far isn't particularly positive so don't expect anything good to come).