A clip and a queue at once
Square and TikTok Shop both push their sales the moment they happen. Neither one waits on a timer.
Square tells us about a counter sale as it’s rung. TikTok Shop tells us about an order as it’s placed. Each one lowers the count the other side works from in under a second. So at 11:05 the till rings the first one and TikTok can now sell eleven; at 11:06 TikTok sells two and the register shows nine. By the time the twelfth goes, whichever side sold it, the other side stops. The fourteen people who wanted one after that get an out-of-stock, not a refund three days later.
Square keeps its catalogue for the register and TikTok Shop keeps its own, and neither reads the other’s. We sit between them and correct each side from the other rather than making either the master. Items pair on SKU, which is the thing to get right first, and TikTok Shop hands orders out but can’t take one in, so if a TikTok order is going to be packed from the counter’s stock, Square is the side that can be told to fill it.