For the first time in quite a while, I received a paper voucher. I tried to add it to my starbucks balance the same way I've done with digital ones, but I got errors (android: something vague, ios: "wrong cvv"). So I went to a shop in the airport, which has the now-pervasive self-checkout. The kiosk had me type everything in (card number, etc.). I used all the same details as for starbucks, but it did work with the physical merchant.

Not sure if this is something peculiar to paper vouchers, or if it was a one-off issue.