If you get some paperboard envelopes and are shipping low-ish dollar raw cards in top-loaders (can't be in graded slabs), then you can ship them for less than $1. And ya, definitely never pay for USPS insurance with collectibles. It's pointless. They'll just take your money and never pay out. Same with FedEx and UPS. They both cap out at $1000 coverage for collectibles but won't tell you that when you buy insurance. It's a scam. Just get private insurance.
Envelopes:
https://www.ebay.com/itm/6-x-8-Paper...g/254808890842
eBay's instructions:
https://pages.ebay.com/seller-center...-envelope.html