how can i save on mailing costs?

i operate a small business selling various types of cards (pokemon, one piece, and soon MTG) , i recently started selling online as well which of course involves mailing. I just got SLAUGHTERED mailing all of these items (14 or so).

Each envelope/order includes:

Card inside of the Top Loader

piece of cardboard cut to size of a card

The top loader and the cardboard inserted into a small bag and sealed (team bag)

placed and sealed in an envelope.

they considered it all packages and not envelopes so i paid roughly $7-8 per item.

The USPS worker informed me that if i remove the cardboard and write "non-machinable" on it, and i do the click and ship option online, it should only be $2-3 per item.

Is there any other way to save on mailing these cards across the country?