Wikipedia's Louis DeJoy; Postmaster General says:
DeJoy received criticism for his decision on February 24, 2021, to award a $6.5 billion contract to modernize the USPS fleet exclusively to Oshkosh, which had previously admitted a lack of expertise in producing electric vehicles and would therefore be unable to make good on President Biden's pledge to make the USPS 100% electric.55,56
55Bloomberg News 2021-03-10 Oshkosh Admitted Weakness on EVs Before Shock Postal Deal
The quote suggests that DeJoy had freedom to choose to whom the contract was awarded. Generally there are procedures for how to award a US government contract based on well defined specifications and procedures, rather than a single individual's personal call.
Question: Does the US postmaster have the power to decide by themselves to whom to award a $6.5 billion contract? Is there a standard government bidding process for large contracts like this?