Having read this a couple of times, I take a step back and ask myself this - is it really worth all this hassle for 2 bottles of tequila? I appreciate this particular brand is not readily available in the UK (or even Spain).

However the US rules on STEBs are the same as the UK, give or take, so tip or no tip, just ensure they follow the simple process of sealing the approved bag and making sure the receipt is visible and dated. What may have helped in this situation is declining ITI, if missing the aircraft was more important than 2 bottles of tequila. They are supposed to give you the option.

And yes, no EC261 applies on this trip, AA isn't even obliged to pay for your hotel in this situation, and almost certainly would not have done so.