First class mail cannot be thicker than 1/4" and technically should not be anything other than fold able paper these days. Anything other than that or any "merchandise" must be shipped as a package. Some post offices do not enforce the rules but the post office at the receiving end may and the person you're mailing the items to might be in for a very big surprise when they get charged postage due at package rates. My local post office would be one of those since they are very strict on things like this and I have received several packages from eBay sellers who do similar to what you're doing and I wind up with a very big bill.