Well the "XL" is the bulkhead row (1st). In 2023, the fee was $80 or 80Euros, I do not recall. The trick was to change the currency to Euros, in this way the overall fare was lower than in US$ (assuming your credit card does not impose fees for conversion).
Yes, there was a Level counter in BOS, and we did check bags, but we had 2 for the 2 of us.
I do not know much about BCN's Level counter as we were also on a cruise that ended in DUB.
And yes, I looked at the TA reviews back in time - they were uniformly awful with few exceptions