I only get $130 with itead for 100x100mm Red soldermask, $2.60/card.
Probably $26 for DHL shipping with itead. No seperate tooling charging.
Would be under 3 weeks, DHL does not have the long delivery time that US Registered mail has in clearing customs.
130/50 = 2.60, not sure how you got $1.63
I've not used PCB cart, or colored soldermask generally, let me know how it goes.
Checking out their pricing:
50 pieces 100mm x 100mm, no colored solder mask, price comes out to $2.85/card.
Plus $57 for tooling, which is over $1.14 per card. I suppose after enough orders that would amortize down.
Be hard to get under $2.60 tho.
Red solder mask: price jumps to $2.96/card, plus $77 for one time fee.
$2.82 for longer shipping.
Smaller card, yes, I could see that would save some vs itead's flat rate pricing for their "up-to" sizing.
I don't see a way except by trial and error to work out what board size you'd have to come down to be under itead's 100x100mm flat price.