I just mailed two postcards the other day, but realized today that when I looked up the postage it was in Euros rather than US dollars, so I inadvertently put insufficient postage on them. Will they be returned to me, or forwarded to the recipient? I am so sad that I messed up my first two!
I put the wrong postage on the first two cards I sent! I didn’t realize that the international post card rate was different from the domestic, so I just put a regular old stamp on them.
When I realized my mistake I panicked and sent extra cards with the right postage, and sent the recipients a message explaining. They were both extremely kind about it–and in the end all the postcards were delivered after all.
PS–Welcome!
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Welcome to Postcrossing 
You put postage on them, so they will most likely be delivered. But why did you look up the postage from the Us in Euros? (If you don’t mind me asking) 
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I would suggest that you just wait and see what happens. If the cards are registered, it’s fine. If they expire after 60 days of travelling, come back here and ask again.
In general it’s important to use the correct postage. Some people have to pay fines for cards arriving with not enough postage. If I receive a card that looks like the sender has been trying to cheat with postage deliberately, I report that sender to the support team. That happens rarely and deliberate cheating to me looks different than what happened to you. In case: if the support team contacts you because of the postage (not very likely to happen on your case, but you never know), you should send them a link to this topic and all will be fine as they will know what happened 
Every now and then when I send many national cards and only one international, it has happened to me that I accidentally used national postage for that one as well and only realized while putting it into the mailbox. It all got through so far, luckily without complains. As it also happens the other way round and national cards travel with international postage accidentally, I guess in my case the mistakes even out moneywise.
Every now and then I have to pay fines in order to get a card handed out, but so far that’s “only” been for national mail.
When I was new and didn’t realize there were size limits for cards (especially for domestic cards), I also accidentally put the wrong postage and found that USPS is very kind…they delivered my cards.
I suggest you wait and see if your postcards are received. If not, you can always resend later. Bear in mind that due to the holidays and Covid, mail is slower than usual.
And if you’d like to make it up to USPS, you can always add on a little extra postage to your next cards. 
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I actually do that, lol.
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I’m fairly certain that if you didn’t put on a return address the cards will just be delivered, if national mail here is delivered with insufficient postage we have to pay a fine, but I’m unsure if the same counts for international mail. I don’t think so to be honest.
Hei there! Welcome!
Don’t worry, mistakes happen. Anywhere, anytime
In my little experience, what’s going to happen to your cards depends on the postal system you are using.
Some countries are stricter than others: in some countries, as long as there is some stamps on the card, they will let it go through.
Some other countries have more controls in place and they might stop the cards. If you have put a return address, they will return it to you. If not，they may destroy the cards.
Bottom line:
always check the postage is correct.
Mistakes happen, we are humans!
See if your cards get delivered within a reasonable amount of time. If not, consider resending… After all, we have 365 days to register a card
Lastly, have fun! Don’t let this ruin your mood!