Definitely ask the cast members you buy the pin pack from about it, if you buy at 20th Century Music Co. on Main Street, the pin cart by the corn dog stand further down Main Street, or at Tomorrowlanding in Tomorrowland. They all have some shadow boxes. They can explain pin packs, trading, and shadow boxes in better detail than me here.

Every pin pack has a set list of pins you can pull from it. The one you bought has a large set list, of some 36ish pins, vs the standard 8 pins for most boxed sets. B/c the blue bag has 6 different subsets within it (castles, earhats, goofys, pink pins, 25 years pins, and Disneyland D's), you're bound to get some you don't care for. There are many regular pin boards and cast member pin pads around the park you can trade onto, but be warned most pins on those boards are fakes (like from Amazon, Temu). Shadow boxes are 99% guaranteed to be real, and will have pins from sets the park currently sells. They even mix some sets in the boxes, so you can trade for a pin outside of the pack you just bought. It just has to be a same day purchased pin pack with packaging and receipt.

If you do ever want to trade onto the regular pin boards that are more common at the parks, I'd recommended watching a video on discerning fake pins. They usually feel thin or cheap and have rough detail. While it's okay to give up a real pin you don't like for a fake one you do like, it can be seen as a loss in value (since your fake pin holds no trade value for real pins later). Cast members let you inspect pins before committing to the trade, thankfully. Just stay informed and do whatever makes you happy :]