Yes, you may report your business activity without Form 1099-K, and with income less than $20,000.
Self-Employment activities are reported on Schedule C, which is found in TurboTax
Self-Employed Online.
The
easiest way to to find Schedule C in
TurboTax is to use the Search box at the
top right side of the TurboTax header. Enter "schedule c", hit Enter,
then click on "jump to schedule c".
This will take you directly to the start of this section where you can set up
your business and enter your income & expenses.
In the income section, answer "no" to the question about Form 1099-MISC and continue to the page titled "Enter Business Income That's Not on a 1099-MISC" (see screenshot #1 below - click to enlarge) or "Tell us about additional income..." (screenshot #2). You may report your sales in the box for "Income or Sales" - the total from your accounting records, since you didn't receive a 1099-K from Amazon.
If you have already set up your business, choose "General Income" from the income summary page (screenshot #3).