"Why use parcel rates if you can still use the Large envelope [flat] rate? As long as it isn't over 3/4" thick and not too rigid you should be OK. "

My considerations were:
¶ I like to get delivery confirmation (and it is cheap with PayPal shipping labels), and the mailpiece must be over 3/4" thick to get delivery confirmation, and flats can not be over 3/4".
¶ I like to sandwich delicate-valuable chips between two pieces of cardboard, which (1) makes the mailpiece over 3/4" thick, and (2) makes the mailpiece too uneven or too rigid for flat (large envelope) rates.
¶ for most weights, the parcel rate isn't so much more than the flat rate.

Robert