It's not that you're being ' pushed into it' it's because Simple Delivery is mandatory for all Private Sellers.
There is no escaping it, once you've been ' converted' there's no turning back.
There are still issues with the new system, one being it's still not possible to offer combined postage.
If postage was purchased though Simple Delivery and you now post all the items together the buyer will receive a refund for the unused labels they paid for, might be worth sending them a message explaining this, and that it may take up to / over 30 days for the refund to appear.
If ' Free Postage' was offered, then it's up to you how much you wish to refund as I'm presuming you added the postal charge to the item price.
Some sellers are leaving a message on their listing, for example..... ' If you wish to buy multiple items, please let me know before buying, I will relist them as one listing so only one postage cost is charged, combining postage is not currently possible. '