I mean the envelope was sealed. And sealed when they arrived, not years later. I think it is also worth noting that there is only 10 years worth of sets where the question of sealed/unsealed even applies. But I do acknowledge that the question, or debate if you prefer, does indeed exist.
The fundamental problem is that atmosperic conditions could easily seal an unsealed envelope at some point during its' long history, with carelessness and not deliberacy as the cause. There doesn't seem to be definitive Mint record on the subject - in a world where people like Roger Burdette exist, we'd know if it were there - so it remains a perpetual 1964 Peace Dollar-type topic. Meat for discussion, which is good for forums.
I don't know what the US Mint sales price was, but I think $2.10 sounds about right. The price I referred to was what most dealers sold them for at some time during the 1970's. Let's be honest, if it wasn't for the silver value, most of these toned and spotted 1960's proof sets would probably still sell for around $3.00. That being said, these sets are still a great way, especially for a young collector, to own and enjoy 50 year old silver proof coins at an affordable price, especially since silver is currently much lower in price than a few years ago.