According to SPS' FAQ customers receive product within "4 to 10 business days." Let us assume the partner in China ships directly to the end customer. SPS has one shipping price for all orders - $6 - which seems cheap for international so for simplicity let's assume SPS makes no profit on shipping.
Their cheapest products -- Mini Squares and Prints -- are both $15 for a set of 48. Each is roughly the equivalent of 4 sheets of 8.5" x 11". If the cost per sheet of glossy thick 8.5" x 11" cardstock is about $0.05 and the cost to print, cut, package and prep for shipping is $0.05 then the cost to print an order is ~ $0.50.
Let's use $10M for their annual revenue. If their average order is $25 (including $6 for shipping) that's 400,000 orders.
$25 (gross revenue) - $6 (shipping) - $0.50 (printing & prep) = $18.50.
x 400k orders = $7.4 million
Let's say each employee costs $100k/year. They have 10 employees, so now SPS is left with $6.4 million.
Their next highest costs might be: marketing, office space, servers, employee computers and professional services e.g. attorney. Add to that other costs of doing business like chargebacks and returns, and call it another $1 million in annual expenses.
Even if I am now overestimating actual profit by an order of magnitude, the founders or investors of SPS are still doing well. This in a market I would have bet was oversaturated. But most startups competing in this space probably do not think to run their supply chain and distribution from China. Not to mention the other areas of the business that SPS appears to be executing nicely on.