According to the online purchasing site the muzzle loader stamp can be printed and will be mailed to you within 10 days, I would assume you can use your printed stamp for proof of having purchased the stamp as it does not come with a separate tag..........but then again I have assumed many things that were not correct.........lol
I bought mine shortly before the season started. Print at home, nothing is mailed to you. I think they did away with the early purchase requirements years ago, but I'm not certain if you can buy one after the season is started. I've seen nothing saying that you can't.
I bought mine after the season started this year. Wasn't sure if I would go out or not in beginning of season, so I waited. And I went and bought it on the 27th of December and went hunting the next day.