Buying Merch from the Box Office On Off-Hours

I'm not sure if this is common knowledge, but I've never bought show merchandise when I haven't actually attended a show so I'm hoping someone can help: Do box offices sell show merchandise at any time during the day that the box office is open, or it is only for that 30 minutes before and 15 minutes after curtain? I remember reading quite a few posts here in the past about going to a theater to buy cast recordings prior to its official release in big box stores, etc. but does it have to be within that narrow window of time? I'm seeing Something Rotten! tomorrow night, but I was hoping to go to a few different theaters and buy merchandise that I saw when I was at the show but passed on at the time. Thanks in advance!

Great, that's exactly what I wanted to know! I was so surprised when I was at Fish in the Dark recently how quickly they packed up the merchandise. It's not yet available on Playbill Shop, either, so I thought I'd give it a shot tomorrow, but now know that it wouldn't be fruitful. Thanks, trpguyy!