Thanks to all responders...
More info: Was using Adblock Plus on Firefox at the time. Filter lists had not been recently updated so that's on me.
As per suggestions I installed uBlock Origin; all filters/updates applied. With uO in place wasn't able to exactly replicate prior results as the package had been delivered. ESET did not pop a message when testing the package tracking anyways, but the 'delivered' webpage may be lacking elements on the 'tracking' page necessary to trigger ESET. Testing outcome: inconclusive.
Will be ordering more from Amazon in the future, and will report back on results.