The short answer is, there's really no special documentation you need, because it's technically not a charitable deduction.
For a business in Iowa, the basic sales tax rate is 6%. Some locations in Iowa also assess an additional 1% “local option sales tax,” which makes the sales tax in that location 7%. Sales tax is not an expense of your business. Instead, it’s a tax you collect from your customers and pass on to the state.
So why does the IRS care so much about employee vs. contractor? Two reasons. One is the penalties they can collect. The other reason is they usually collect more taxes when someone is treated as an employee, because a contractor can take business deductions which directly reduce taxable income.