That's an SF-86. You fill that out for clearances as low as Secret. The bar for a Secret is roughly "Are you blackmailable? No? Here's your clearance.". You can ship Secret material through the USPS if you ship it as registered mail. There's no need to hand-carry it.

There are many, many things that are classified as Secret that are either overclassified, are classified as such to cover someone's ass or prevent embarrassment. Secret material can be sensitive stuff, but should never be serious spy shit. If it is, someone seriously fucked up their classification guide.