Just another simple, time-saving macro. Calculating postage on the USPS website involves a lot of clicks and typing. I made it simple, in part by restricting the options to regular and large envelopes, which comprises 99 percent of mailings for which I need to calculate postage. The other thing that makes this work smoothly is a digital scale, which I have because I took up bread making during the pandemic. 
By examining the URL components and playing around with the USPS website, you should be able to add options to those offered in this macro. Again, the macro was optimized to the most use-case scenarios for me.
This macro is uploaded in a disabled state. Weight and origin/destination Zipcodes default to previously entered values.
POSTAGE CALCULATOR.kmmacros (6.6 KB)
The resulting page displays a variety of postage amounts depending on whether you have an official USPS Priority envelope or other types, etc. The required first-class postage to simply stick on a regular size letter or large flat envelope is going to be at the bottom of the page:
