This is a flat, substabtial interface with the 10th Amendment’s allowance of states to manage and run elections. If a state allows elections such that they aren’t denied on account of race, gender, and people under 18, they can be run however the states deem. Requiring someone to register with the federal government is not a power of Congress
That sucks but
it doesn’t necessarily prevent states from mailing ballots using UPS or FedEx, etc.?
Can you afford to put a UPS label on things when a stamp would normally do? I can’t.
LOL I’m not the government
Only the USPS can legally postmark them, with rules limiting the ability to send them to voters via other services.