How Shape and Size Affect Prices
In First-Class Mail, postcards cost less than letters. Letter size mail costs less than flat-size mail. Parcels pay the highest price due to the cost of processing and delivery.
In USPS Marketing Mail, there is no postcard price. You can mail postcard-sized pieces as USPS Marketing Mail, but you'll pay the same as a letter. Letters cost less than large envelopes (flats). USPS Marketing Mail parcels (including tubes) are charged a higher price due to processing costs.
Can you get a better price by re-shaping your mailpiece? Possibly. You could fold your large envelope to be letter-sized, or you could redesign your parcel to qualify for the lower flat price.