I am not an attorney and any comments I post are not intended, nor should they be construed, as legal advice. If you need legal advice, please consult a legal expert who is familiar with the area of legal expertise you need.
I found this article, Is DNA Left on Envelopes Fair Game for Testing? https://www.theatlantic.com/amp/article/583636/, in a DNA search on my least favorite search engine. It’s sub-title is
The genealogist’s dream of testing old, spit-laced artifacts is coming true—but raising questions about who controls dead people’s DNA.
The above is somewhat easy/complicated to answer. In the U. S., state law determines the answer. In some states, there are no restrictions on DNA testing of somebody else’s DNA outside what laws may apply to law enforcement and prosecutors. In other states, if a person said no to DNA testing while living, that cannot be overridden by any heirs who want the DNA tested. In other states, the heirs can override the dead person’s final wishes. For other countries, the national, county or county equivalent, or local laws would determine if the DNA could or could not be tested.The article mentions MyHeritageDNA, LivingDNA, TotheletterDNA,
A separate issue that often isn’t covered is you may only get one shot with DNA testing an artifact as the portion of the artifact used is destroyed in the process. Also, you may not get enough viaable DNA to test. If that’s not bad enough, this kind of DNA testing is new enough that it’s expensive and less reliable than it will be in a few years.In most cases, you won’t get a refund if a company can’t get DNA from your sample.
If you have a potential DNA source like a licked stamp, best to research how to preserve any DNA you may find. You also need to find out what the law says in your area if the person made it clear they didn’t want their DNA tested.