There is nothing to stop you buying tickets online using the railcard discount - provided of course you get the railcard before you start using the tickets! - so that might save you time at Hayes.
If need be you could use contactless as far as Reading and use the interchange time there to get your railcards?
Pausing at Reading to buy the railcard there is
exactly what I plan to do as a back-up plan if it turns out I don't have time to buy it at Hayes & Harlington! Great minds think alike.
I would be more than happy if you wanted to have the physical card sent to me, to give you my address, and then post it to you via airmail to wherever you live.
I would send you the proof of the posting from the Post Office here in England, and in return just request that you make a donation for a similar amount of the postage to a charity of your choice in your country.
please PM me if you would like me to do this.
Thanks for the offer, but I don't think I'd have time to do this, as I'll be traveling fairly soon -- and international post takes too long.
My contingency back-up plans are these:
-- Use Oyster to go from Heathrow 2&3 to Hayes & Harlington
-- If I have plenty of time until my next train at H&H, then buy the Two Together card there, and use it to buy my now-discounted ticket out to the west country
-- If I only have a few minutes until the next train westward from H&H, then I'll just use Oyster to ride the Elizabeth Line to Reading, and debark there.
-- If I then have plenty of time until my next train departing from Reading, then buy the Two Together card
there, and use it to buy my now-discounted ticket out to the west country.
-- If I again only have a few minutes until the next train westward from Reading, then I'll just pay full price for my first ticket, and buy the Two Together card once I've arrived at my destination station.
Paying the discounted fare is nice, but getting to our destination as promptly as possible is the highest priority, even if it might cost a bit more!