Thanks for the comments. I talked to Mileage Plus customer service today. They confirmed that this was done in error and that I should not have been charged miles or copay. They redeposited the miles and put in a request for the copay to be refunded.
Possibly of more interest; my opinion of PP is that it's an improvement over EP but would have to be priced right to make it worthwhile. I would rate PP is to EP as EP is to regular economy, which is similar to my previous experience on Singapore Airlines PP equivalent. It's not a radical improvement for a long haul flight. YMMV.