In any case, even if the process was terribly corrupt, I'm not sure why that would have you so bothered. Last sentence from the USPS press release on Oshkosh winning: "The Postal Service receives no tax dollars for operating expenses and relies on the sale of postage, products and services to fund its operations." https://about.usps.com/newsroom/national-releases/2021/0223-...
Both results are on the first page of a google search for 'usps truck bids'.
The Postal Service has a legally enforced monopoly that it abuses to deliver literal metric tons of spam into our mailboxes every single day.