I don't think you can purchase prior-year Audit Defense protection through TurboTax since you already filed, but you may be able to purchase it directly from the company that provides that service, i.e., TaxResources Inc., aka taxaudit .com
Here's their website. You'll be able to find pricing, FAQ's, and contact info there, and a link to purchase.
https://intuit.taxaudit.com/
To get the TurboTax affiliate discount be sure you use the URL that has "intuit" in it, i.e. intuit.taxaudit.com as shown in the link above. You'll see the TurboTax logo on the page there. Their site without "intuit" in front of the domain has a different price and no TurboTax logo.
After clicking to get coverage, I do see prior years listed in the products for purchase there. I would phone them first to discuss it, however, especially since an amended return is in the mix.
Here are a couple of FAQs copied from their website:
" Can I purchase Audit Defense Memberships for prior years?
You can purchase Audit Defense Memberships or all prior years that may be
subject to audit. For federal income tax audits, the time period is typically seven
years. The states have varying time periods."
"
Is my amended return eligible for membership?"Amended returns are also covered under your membership in accordance to
your membership agreement."
