That Holley carb would have fit in a large $19 flat rate box, but surprised USPS accepted it at all? Those 4 questions they ask about anything "liquid, fragile, hazardous, or ????" would make the used Holley carb not allowed. Unless it was new, or freshly rebuilt, so no gas smell. They have sniffers at their sorting stations, and usually gasoline smell will kick a package off the line.