USPS Post Offices have a stamping machine to stamp envelopes based on weight.
Because envelope stamping machines print stamps on envelopes faster rather than manual stamping by a human being.
Let us discuss What Does Hand Cancel at Post Office Mean?

What does It Mean to Hand Cancel at Post Office
Generally, stamps are printed by a machine on every envelope.
Sometimes, postal workers stamp manually on envelopes in US post offices.
This manual stamping process is known as Hand cancellation at USPS.
In short, we can say that Hand Cancel means stamping done manually by a postal worker in the US post office.
How much is the hand Cancel charge at the US post office?
According to USPS’s official website, $0.44 per envelope is the hand cancel charge.
It means you have to pay an extra amount of $0.44 for hand cancellation of your wedding invitations.
What is the difference between hand cancel and machine mall?
Mostly, mail envelopes are cancelled by automatic machines at US post offices.
These franking machines do not print stamps on inappropriate-sized envelopes.

Postal workers have to stamp these large-sized envelopes manually. This manual process is known as Hand Cancel.
What does a hand cancelled envelope look like
See the below sample image of hand cancelled envelope for reference purposes-

FAQs
Do you have to hand-cancel wedding invitations?
If your wedding invitation cards are large, you have to hand-cancel these cards at any post office before posting.
Does hand cancelling cost extra?
Yes, you have to pay $0.44 extra charge for hand cancel.
How much does it cost to hand cancel mail
$0.44 per envelope is the extra charge to hand cancel of mail envelopes.
How long does hand cancel mail take?
A least one week.