No no no!
You can't really try underwears at store. Think it as this way because as you're putting on your boxers to try them on, then the next person puts it on. It's like your skin's touching it and then another, that would be dirty and a bit off. Look at your underwear that you're wearing now and find a size at the shop that looks like that. See if the underwear looks like your size, buy it and take it home. You can also do this. Put your underwear that you're trying and then put it over the one you're wearing and try it on. Hope I've helped
No don't do it
Things that come in packs, generally are not supposed to be opened. A lot of the time, the cashier might tell you can't purchase it because it's been opened and they can't sell that merchandise.
I wouldn't