I would say 2 or 3lbs. Not sure what that would cost to post to America.
Julie in Surrey and I have mailed various things including books and model trains both ways between Surrey and Missouri: it recently cost just over $40 to send a 2lb book first class from here to there in a minimum allowable dimension 4 inch deep box.
Modeling is currently being financed off Social Security Disability and unlike those people you hear gossip about I've not yet figured out how to get rich off that income.
I would absolutely love to build their 1/72 scale series, Puffer, Drifter, and somewhere I think I've seen a 1/72 tug, to go with my supposed 1/72 scale rubberband powered balsa submarines but there is only the one of me and health is a bit inconvenient.
Wish Mountfleet's product listings were more open and up-front about what boat is what scale,
https://www.mountfleetmodels.co.uk/the-range/ A few boats give scale up front but all the rest are a rather irritating buried treasure adventure.