In most cases (in my experience) ebay recommends the size and weight of a parcel, so that isn't a problem (in most cases).

You can't upgrade to a more expensive service, other than paying for the whole cost of that yourself - taking the delivery out of Simple Delivery.

Ebay is responsible for the delivery of Simple Delivery - whether it arrives on time and if it gets damaged. Ebay should the refund the buyer if lost/damaged without any input from you.

You can't buy Simple Delivery at the post office.

You can list without Simple Delivery on items ebay allows you to opt out of.

You shouldn't use Simple Delivery on items which are prohibited.

https://www.ebay.co.uk/help/selling/posting-items/setting-postage-options/simple-delivery?id=5575

https://www.ebay.co.uk/sellercentre/postage/simple-delivery