> but ordering stamps... on eBay

OP was ordering US stamps to include _in_ the letter, on an SAE (self-addressed envelope) they were sending _from_ the UK, so that the FSF could reply (from the US) using said stamps.

As a millennial myself, I have no idea where else I'd look for <recipient country> stamps should I want to include them on a SAE I was sending to said country, so that they recipient wouldn't incur the cost of replying to me.

I don't find looking on eBay particularly strange, though I'd do a quick search for alternatives first.

> I have no idea where else I'd look for <recipient country> stamps should I want to include them on a SAE I was sending to said country

I would try to buy them online from their post office. For the USA, there is https://www.usps.com/business/postage-options.htm:

“Print Labels Online with Click-N-Ship

With your free USPS.com account, you can pay for postage and print just one label or a batch of shipping labels online”

Germany has (https://www.iamexpat.de/expat-info/germany-news/deutsche-pos...):

“You simply need to open the app, select the appropriate postage service, tick “Code for labelling” (Code zum Beschriften), and pay with PayPal. You will then immediately receive a code, consisting of the letters #PORTO and an eight-digit string, which you must write in pen in the top right-hand corner of the envelope or postcard. Then, just pop it in the post box, and you’re done! The code is valid for 14 days and can only be used for Germany-bound mail.”

That 14-day limit may not be a good idea for this use case.

Offhand, I don't think I've ever mailed an International letter or package.

Is return postage something that, normally, my local post office would help me with? E.G. do they have some method of marking or adding post to a package that would be accepted globally (or at least within the destination country)?

Sure but, on the other hand, this was overly kind of him. In general, unless it is explicitely requested that you must provide a stamped envelope for the reply the assumption of snail mail is that each side pays for its own envelopes and stamps.