Once the tires have flat spots, it's too late: the tires are deformed and need replaced. My port of entry was Philadelphia, and I believe the shipping time from Korea is 30 days, or so I was told.
You'd be surprised as to how much car processing facilities can do. Accessories are typically installed at the port of entry, requiring a shop to do that. They wash cars... etc.
Here's what Philadelphia advertises for one of their car processing facility:
Accessory shop accommodates 125 vehicles;
Auto-washing system handles 125 vehicles per hour;
Computer tracking system follows entire process;
Designated Foreign-Trade Zone
And for another one also in the port of Philadelphia:
108,000 sq. ft.
Two car washes, able to service 200 cars per hour
VPC capable of processing over 1,000 cars per day
Specialty heated floors – To assist productivity in winter weather conditions
State-of-the-art body shop with two double prep stations and two joined double paint booths
Here's a small section of the port of Philadelphia from Google Maps (it's way bigger than just this screenshot). Notice the big building with the Hyundai and Kia logos:
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