Couldn't mail my packet on Mar 7 as planned but was able to mail it on Mar 30 and was received by the Elgin Lockbox on the 31st. I guess that still makes me a March filer. Here's my timeline so far:
Mar 30 (Thu) - overnighted via Fedex
Mar 31 (Fri) - delivered per Fedex tracking
April 4 (Tue) - got a text from my bank at 23:44 that my cc was charged
April 5 (Wed) - got a text from 87247 at 13:25 that USCIS received my packet on Mar 31 with receipt number starting with IOE.