Where can I send a box for pick-up in Braemar? My guess is the post office won't be open, since I, like most people, will arrive Saturday or Sunday?

The post office in Braemar (and quite a few other places in Scotland) is part of the local Coop shop, you'd have to contact directly to find out if you could get hold of your parcel poste restante on a Sunday. However, where are you planning to stay? Most B&Bs, hotels and hostels will likely keep a parcel for you if you have booked. I have also previously sent a parcel to the campsite, although that was under different ownership. I imagine the new owners would also be amenable.
Contact details can be found via Google 😊

Thanks Louise, you are very helpful!

You could also try Braemar Mountain Sports as they are open seven days a week.

Good idea, I didn't even know about them.