This is more informative, rather than a question. When I went to Scotland in May, I was there for 3 weeks so it made sense (to me, anyway) to purchase a Historic Scotland membership, rather than the Explorer pass. For one thing, it wasn't that much more expensive, and for another, it supports a good cause. And lastly, because of all the HS places I went to, I actually saved money especially when you consider the 20% discount in their shops and 10% discount in cafes.
I wanted to buy it at the first place I went to, which was Stirling Castle. Because you have book ahead for Stirling (well, it's recommended), I was able to book by going to the booking page, clicking on "Historic Scotland Member" as my option, picking the date and time I wanted, then adding it to my basket. When you check out, it will ask how you want to collect the ticket, and I just chose the "Collect on Arrival" option. You will then check out (no purchase required as it has a zero balance) and will get an email confirming your date and time.
Then when you get to the site, you show them your booking info and let them know you want to purchase a membership at that time. You pay and they give you a temporary pass that is good for one month. You then use that for any other sites you see while you are there.
I assumed that they would mail me the permanent pass to my home address but when I hadn't received anything by the end of June, I emailed HS and asked them about this. I sent them my information, including the barcode number on the temporary pass. I received a quick response from HS telling me that US members must pay €10 for postage to receive a physical pass. However, he told me that they have recently implemented a digital pass, which you can access from the HS app. He then gave me my member ID number via email and I tried it on the app, and voila - I have a digital membership card that I can use until May 31, 2024.
Many travelers here will probably not worry about it if they don't have travels plans to go back to Scotland within a year. But I will be heading back next April to France and England and may take a short detour to Scotland while I'm there, so it's nice that I will be able to use this.