1990 United States - USSR Joint Issue

I have a question regarding the Sea Creatures joint issue between the US and USSR in 1990. I recently received a US parcel in which the Soviet version of the Sea Lion stamp was cancelled and used as postage. Is this normal for joint issues to be accepted from another country as a regular stamp?

No. It must have been overlooked by the post office at the point of mailing. Postage stamps are only valid in the country they were issued in (even with joint issues).

Here's a more modern one (Miles Davis, Edith Piaf) that was a joint issue between the US and France. Note that the link below is for the French version of the stamp that the USPS is selling but is clearly noted that it is not for use as postage in the US:

https://store.usps.com/store/browse...r-stamp-rate

Thanks for the response WT. It appears that the postal clerk did exactly as you said and just cancelled the stamp as it was on an envelope that I assume would not be machine sortable. Wish I would have kept it on piece now instead of trimming and soaking.