Most carriers have femtocell options for improving indoor coverage.

It's the postal carrier I worry about.

We have multiple carriers that we shop around with and some companies are better than others depending on the needs of the client!

I would run a campaign with both and test them for your market to see if the improved response rates justifies the cost.Depending on the type of list your mailing, you can even do carrier route mailings.

On the other hand, if it's just a stick-built structure and there's a cell tower stocked with all the carriers right nearby,  then by all means, have at it.  

If they don't come up as non-deliverable there is a problem with the letter carrier and it should be reported to the ombudsman at the general headquarters of the USPS. 

Our current setup is a Toyota RAV4 crossover with a roof rack, a folding cargo carrier, and two utility trailers.

If you're calling your normal carrier, press them on what they cover and what they don't.

(And your concern about nuclear bombs being walked over from Mexico can also be solved because the bomb carriers will not be able to hide among thousands of non-bomb carriers.)I actually think the war against illegal immigration is quite similar to the war against drugs.

We have 20 carriers so can pretty much insure every situation.