I don't use cardboard either, I put things in ziplock bags and squish all the air out then seal it. If you are sending clothing in an envelope you need to make sure it is waterproof too.

You don't have to use padded envelopes either, I have a stock of ordinary envelopes and shurtuff mailers, as long as they drop easily through the slot they can go as a large letter.

If you have to help them through then they are a parcel.

For things that will not go as letters your most economic way of posting is click & send with the ebay branded satchels, for anything worth up to $50 ($100 with signature required) they are cheaper than regular small parcel or the red satchels and have Paypal seller protection.