So you are buying BCN - SFO for $400 or so. TAP has a single fare for that. Many airlines (BA, IB. AY, AF, KL etc etc) only sell fully flexible tickets on single fares.

Now I'm going to use BCN-LAX here, just to illustrate the point about using Iberia. Strangely Iberia doesn't fly direct to SFO but does to LAX. They have a low cost spin off, Level, and that has reasonable single fares. And looking at a random date in April the lowest fare is $500 for economy + 1 checked bag, and premium economy - which is a big nudge upwards on your TAP seat - is $550. And that's a direct flight too. If you want to stick to Iberia metal, overall a better airline than TAP, the single fare is a whopping $3000, because they only sell fully flexible tickets as singles. But you can buy a return ticket on the same outbound service for $485. So you'd buy that return ticket, travel as planned BCN to LAX, but don't use the return fare. There is no come-back or penalty for doing that. Or better still, set the return date as far off as possible and who knows, maybe you do end up using the return after all. They may also alter or cancel your return service and so you can claim an involuntary refund, probably about $180, making the net fare $300. That's not guaranteed.

The point I'm trying to make is that when searching for options on a one-way ticket, when it comes to crossing the Atlantic, and many other routes, it's important to check the return fares even though you won't use all the tickets.