I'm assuming your main reference is to identify cards as goods rather than documents, but nowadays the term has a wide interpretation per Wikipedia and if their opinion is accurate then cards qualify as kinds of documents. (Not that I have ANY interest in this topic.)

From USPS website … I’m not a lawyer nor do I play one on TV, but I would definitely say the standards and regulations are ambiguous.

https://pe.usps.com/text/dmm300/601.htm

600 Basic Standards for All Mailing Services

3.3 Odd-Shaped Items in Paper Envelopes

Pens, pencils, key rings, bottle caps, and other similar odd-shaped items are not permitted in letter-size or flat-size paper envelopes unless they are wrapped within the other contents of the envelope to streamline the shape of the mailpiece and prevent damage during postal processing. If an odd-shaped item is not properly wrapped, it could burst through the envelope and cause injury to employees and damage to USPS processing equipment. Odd-shaped items that are properly wrapped within paper envelopes and sent at letter prices may be subject to the nonmachinable surcharge under 133.1.5 or 233.1.4 for First-Class Mail letters, or the nonmachinable prices under 243.5.5 for USPS Marketing Mail letters. Certain types of odd-shaped items, when properly wrapped, are permitted as automation letter-size mail subject to the standards in 201.3.10. Flat-size automation mail is subject to the uniform thickness requirement in 101.2.4.

I can confirm that this works. Sandyrusty taught me what to do, and I mailed two large envelopes to him today - was $7.33 each. 216 total cards. Less than half the price of mailing them in a package.