I found recordable envelopes online and covered them with red paper. My son recorded a message “Hey Tom! You’d better come to my party or else!” or something like that, so when the kids opened the invitation it yelled at them, just like Ron Weasley’s Howler.
The Howlers were tied with string and wax seal, then put in 9X12 envelopes with an owl and scroll mailing label, to emulate “Owl Post”.