I've looked this up previously. Stacy and Debbie are correct. There's an extra $.20 nonmachinable surcharge. It's covered on this page, section 2.1
Criteria for Nonmachinable Letters
A letter-size piece is nonmachinable if it has an exterior surface that is not made of paper ...
In Lydia's case, USPS must have taken one look at the gorgeous cards inside her clear envelopes and decided they just couldn't bear to slap Postage Due stickers on them.