Hello! Does anyone know the cost to send a square postcard internationally from the USA?
All postcards and one ounce letters internationally are $1.45! You could pay an additional ounce if your worried, but I’ve sent an oversized postcard for $1.45 and it arrived fine : )
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Perfect! Thank you so much. I wondered if I should stick any additional postage on 
Actually, square postcards, as with square envelopes, are subject to a 40¢ non-machinable surcharge.
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Hmm, I found some postcards online that indicate they have rounded corners. When I try to see if these are okay with USPS, it seems that postcards are supposed to be rectangular with angular corners. Does anyone know for sure?
Rounded corners are treated the same as angular ones.
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I think you shouldn’t be worried about rounded corners. I’ve mailed shaped postcards WITHOUT envelope, written and stamped as a regular postcard and they’ve arrived just fine. Like those 
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Sending an extra card abroad? I’ve attached the card with washi tape all around the card to ensure it doesn’t fall off. As long as it I weighs [1 oz.], you are good to go. (International Mail Services & Shipping Rates | USPS) See link for price.
For domestic, I’ve sent a postcard 5x7 which cost me the first class postage stamp rate, not the postcard stamp rate.
USPS has a postage calculator on their site you can use.
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Oh, those are cool!
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Strictly speaking Kliban’s Cat was not a postcard but kind of cardboard tag and of smaller size that standard postcard but I mailed it as a card, travelled without trouble 
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Yes, attach the second card to the first one corner to corner with washi tape so it easily comes off when peeled.
I am sending a square postcard today (slightly over 5x5 inches) and checked on the USPS site: it does require additional postage! I’m glad I checked first.