I’d deem it extremely unlikely that US customs processing or the express carriers will be able to handle that overflow in anything like a reasonable timeframe.
The carriers do already have practice ramping up and down for Christmas and Chinese New Year so it seems plausible they could absorb significant extra volume in whatever time it would take to negotiate the leases on the extra flights they use during those times.
For that matter customs processing also has experience managing the same surges.
I'd believe we might have some sort of own-goal planned for customs that could hang things up.
Nothing more American than the government attempting to kill competition so multi billion dollar corporations can get richer