The term you're looking for is "Notary". There are plenty of them around, and they're required to execute high-dollar contracts like buying a house or wills. E

The term you're looking for is "Notary". There are plenty of them around, and they're required to execute high-dollar contracts like buying a house or wills. Every state has its own requirements to become a notary(with one requirement being posting a $10k bond or purchasing insurance, so you have something to lose if you make an egregious mistake). You can require counterparties notarize any documents you'd like, and reject anyone who declines. And you can do this without needing to change the law to increase the scope of responsibility of the USPS. It's illegal for the USPS to offer notary services, although they're often located near notaries: UPS Stores usually offer it.