I believe that if a Package is Sig Reg, and there is No One to sign for it that it is held for an amount of time by the carrier so the person can pick it up at the Post Off, Fed-Ex/UPS Location, etc.
I'm not sure how long they will hold it before it gets sent back to the Person/Company who shipped it.
I believe they give you 3 notifications, but it's been a long time since I had one and the rules may have changed. I did find that I was wrong on the USPS and PACT. Originally, PACT shut USPS down in tobacco shipments. However in July 2011, USPS made changes that would allow a vendor to comply and ship using their services. That still wouldn't help with shipping costs internationally but it's a step.
OCD may have an answer here, but my gf has a business selling books. When they don't get delivered for one reason or another, they get sent back postage due. Now this is media mail which is somewhat cheaper, but it's an additional cost. She only gets a single notification as I recall (she's sleeping to verify), but after a few days it gets sent to an unclaimed area of USPS. What they do with the packages is unknown to me. One way or another it increases the cost of products sold and I'd expect adult sig required costs to be more than the $5+/- additional service charge.
And speaking of how $5 is not made for the convenience, I'd agree from my own perspective. However, for a lot of people that's a heavy charge, that's equivalent to a little less than a 6% sales tax on a $85 order. Adding $5.80 priority mail, small box charges you're at around 13% additional cost for internet delivery and your close to $100 order. However someone with a limited budget isn't going to be able to place many $85 orders. What if they need a couple bottles of e liquid, say 60ml of 24mg nicquid. That's a $16 order. $11 in shipping and adult sig doesn't look so inexpensive anymore.