Current USPS Mail Regulations https://pe.usps.com/text/dmm300/dmm300_landing.htm
Current USPS Rates https://pe.usps.com/text/dmm300/Notice123.htm
Addressing Standards for flats https://postalpro.usps.com/node/1125
Create a Postalone account https://reg.usps.com/entreg/RegistrationBusinessAction_input
USPS Business Reply Mail Tool https://postalpro.usps.com/mailing/abrm-tool