In 2019 (pre-COVID), Walmart imported close to 900,000 ocean containers worth of cargo (20-foot equivalent units) into the US.
We're guessing you shipped fewer than that.
Size matters in ocean shipping, and the big shippers leverage volumes to command better rates and better access to capacity. And better service – they may actually get their calls returned, while your calls to carriers and large forwarders go unanswered for days.