With regard to sending postcards to Russia and just writing “Via Germany” or “Via United Kingdom” by the address, it is necessary to attach postage to compensate the Deutsche Post or Royal Mail for their part in delivering the postcard. If you wish to do this method, make sure to check this forum thread- Prices of stamps for postcards in all countries/territories (wiki) to be sure of the current postage rate for the respective countries. Be sure to add the German/British stamps below the USA stamps so that they aren’t damaged by the USPS postmark.
The easiest way to do this is with the Deutsche Post, you can navigate to this page on their website- https://www.deutschepost.de/en/b/brief_postkarte.html and select the €0.95 option for international postcards. Then you just put in your payment info and can print the stamps out at home, if you print them on normal paper you can use a glue stick to attach them, or you can purchase sticker or shipping label paper from the office supply section of any store, then print on that and stick it on the postcard.
This method allows you to instantly purchase and print the stamps, but you can of course purchase real stamps online and use those too.
My favorite method for purchasing stamps from the Royal Mail is by buying them directly from their official shop on eBay. They have a variety of stamps available at face value and include free shipping to the USA. Royal Mail Stamps on eBay
This page has a ton of different products to navigate, including many that are geared more towards collectors instead of people who just want to buy stamps for postage. In general, the products labeled “presentation pack” or “prestige booklet” are the ones that are as close to face value as possible.
The international postcard rate is £2.20 at the moment, but postage rates are slated to increase later this spring and I’ll update this when they do.
There are 2 products in particular that I like best for these purposes- King Charles III: A New Reign Royal Mail Coronation Stamps Presentation Pack | eBay
This one includes 2 £2.20 stamps (international priority stamp) and 2 1st Class rate stamps (worth £1.25 each, using both of them slightly overpays the international priority rate)
Official Royal Mail Christmas 2023 Collectors Sheet | eBay
This one includes several international rate stamps, several 1st class stamps, and several 2nd class stamps as well. 2nd class stamps are worth £0.75 each, so using 3 of them is worth £2.25, which barely overpays the postage rate as well.
You can also purchase Royal Mail stamps on the secondary market, but keep in mind that regular issue definitive stamps that do not have a barcode are no longer valid for postage in the UK, but you can refer to the Royal Mail Stamp Swap Out scheme website for more details on that. Special stamps and Christmas stamps without barcodes are still valid, this image should help.
Stamps like these with either Queen Elizabeth II or King Charles III like these are still valid if they have barcodes-
Using the “Via United Kingdom” method is great if you’re only sending one or two postcards, but if you are sending more than that, it is better to only use Royal Mail stamps on the postcards and put them all in a larger envelope to mail to the Royal Mail special postmark center (info for that is described earlier on this thread) because that way you only have to pay for the US Postage one time, to get it to the Royal Mail postmark center, instead of paying it separately on each postcard mailed.
I hope this makes sense, if you have any questions please feel free to ask and I will be happy to help!