There is considerable confusion and discussion in online forums re meeting entry requirements for Covid testing into Portugal.

We know we can pay an exorbitant fee ($275 ) at the airport.

Can we use a free Walgreens NAAT test ? In theory this would meet the 72 hour in advance requirement.
Would this Walgreens NAAT test be acceptable by United Airlines prior to boarding?
Would anyone have definitive proof of either acceptance or rejection by United of a Walgreen's NAAT test ?

There is an alternate offered by Vault Health ($79) which involves a spit test that is monitored online via Zoom.
Would anyone confirm that such a test administered online is acceptable to Portugal for entry.
There is chatter in online travel forums that Portugal does NOT accept a spit test and also that it does NOT accept an online test even when monitored via Zoom.

We plan to depart in 6 days. Appreciate a rapid response.

Do not use a monitored self test! That is not acceptable for entry. I haven’t yet gone but I’ve also been following the online threads and many people have entered with a NAAT test from Walgreens. Please know that I haven’t gone yet, but so can’t swear that it’s okay with United, but so many people have used them from what I’ve read, that I am confident you’ll be fine.