Purism has thought of this and offers an "anti-interdiction" service to counter it [0]. This includes tamper evident seals and glitter paint on the screws (for laptops).
Thank you; good to know that they have that.
My assumption would still be that if the intelligence services want access to your private information or communication on the device that they will get it (without it be evident); whether it is a OS zero day or other means.
I also read through the link you provided on the anti-interdiction server, it only says laptop. I assume that this has expanded to the phone. (Did not go through the phone order process to find out.) Also, looks like one negotiates via PGP email on what one wants; so that is why I assume that there is no pricing on it.