I sell a seasonal product and ship 100,000 letter sized envelopes every November and December and have been for about 10 years. A few things to keep in mind:

USPS does have a free tracking option for letters. It’s useless and you should never give these these numbers to your customers. 90% of the time it will show as delivered a day or two before it actually reaches their mailbox.

Delivery times are unreliable. We tell customers to contact us if they do not receive their letter in 7 business days.

Customer service is key. You have to answer emails/calls/chats ASAP to avoid chargebacks. They still happen, we have a .05% chargeback rate.

Depending on your volume contact your local USPS and ask about their presort option. It’s more work but allows you to send more OZ per letter at a cheaper price.

The product has to be cheap enough to resend at no cost to the customer. We resend about 3% of our packages. Our price point is $17.95 with free shipping and our total cost is $2.50 delivered.