Although there is no eFiling for I-539 - 回複:回複:pjiang, radiology
I think you can register an account at USPS, then print a pre-paid shipping label for Priority Mail (flat rate envelope) which costs $4.60.
There is a date printed on such shipping label which will qualify you to the lower fee.
https://ecap-ws-prod.usps.com/entreg/login.do