Here’s some tricks I used early on when I felt just like you do… omg, my transactions will be all screwed up!
I always made a copy of my Tiller Sheet just in case and then…
First, I categorized my Amazon buys with a category of ‘Amazon’. Then, once I get the CSV, I make these selections:
Then, when I hit the button, the result is my transaction sheet with ONLY the new adds showing.
Then, I color fill the lines.
And fill in my categories.
Next, I turn off the filter to get to my full translations.
And now I can easily spot the changes that the importer made.
Early on, when I screwed stuff up, it was really easy just to go in and delete or fix the added (colored) lines.
I was so hesitant and afraid of screwing stuff up in the beginning. But, now, I no longer even color the lines. I, too, support Amazon fully with nearly every one of my nickels 
so the importer is probably one of the most important parts of Tiller for me.
If you break your sheet, go back to the one you saved and try again. I have a folder called Tiller Backups. It’s a safety net that has saved me from my own experimentation many times.