If you choose to take a deduction, it's on you to keep the relevant documentation for that deduction for the period proscribed by law, generally 3 years. It's been a well-established principle of tax law for several decades, and there's really no excuse now that records can be easily and trivially digitized.

> do you know how many of them could readily provide carbon copy receipts for every single deduction they claimed in the last X years?

So on what basis did they take those deductions in the first place, if they didn't have the documentation to figure out what their deduction was in the first place? Did they just guess? Or are you saying they had it for one tax season and then destroyed it?

Either they had it and destroyed it, lost it or did what lots of small businesses do and had a credit card or checking account only used for business, then referred against the statements to determine deductions (sans receipts)

Is an e-mail copy receipt sufficient for the IRS? As a small business, we travel a lot and obviously book travel online (as well as make a number of other business purchases online).

I've never even considered that the e-mail receipt or purchase history on Amazon.com might not be sufficient documentation. Is this something I should spend the next half-hour researching, or am I concerned over nothing?

IANAL, but looks like it's addressed on page 9 here: https://www.irs.gov/pub/irs-irbs/irb97-13.pdf

I'd be shocked if it wasn't, since invoicing purely by e-mail is pretty common these days.

Receipts can be trivially documented by taking pictures with a cell phone at the time of purchase....

In my experience bank/cc statements have been sufficient documentation for smaller or recurring expenses.

Anyone with W-4 and investments in a major mutual fund company probably doesn't have a problem complying with the tax law.