'I don't recall receiving the email. I suspect many sellers might have misunderstood it and assumed the services had been added, and the seller could turn them on if they wished.' 

Yes - even though I did get the 'Updates to your shipping option' in eBay Messages, the bit about InPost 'enabled already' & 'if you'd like to opt out'  were in the last paragraph, after all the blurb & details about InPost.

I suspect quite a few people would have either not noticed the message, or just skimmed the first bit, thought 'not interested' - and missed the important part about already being opted in. 

Surely it would have been better to allow people to opt in of their own free will, rather than potentially cause all sorts of problems for those who only discovered InPost had been added when an item sold?!