Actually yours, if not tracking is put on your items, can be considered a "high risk" account and eBay/PayPal can put holds on your fund for up to 21 days for all transactions and can put selling limits on your account.
And if a buyer files a item not received dispute you will lose and be forced to refund.
If there are issues with disputes or FB because you refuse to put tracking on your items or if your buyers file reports etc then yes your account can be suspended too.
These are not new policies. Been in place for years now.
You should read the Help pages for sellers over on eBay and the PayPal policies. These are not Auctiva issues.
As for your original questions it has been answered. Tracking is not available on FC letters. It is only available on FC parcels and that includes thick envelopes.
You cannot get "tracking" for FC letters on eBay, PayPal, Auctiva or USPS.