To keep the item slide until you die, you have to read the brochures before giving the last ingredient of the hangover remedy to Goodsoup, and end the conversation before Guybrush is done reading the brochures.

The problem is that, sometimes, after ordering the drink, Guybrush will continue reading the brochures before Goodsoup says "Sure". We think that whether or not this happens depends on the exact moment you give the final ingredient to Goodsoup. If you give it at the right moment, when you order the drink, Goodsoup will say "sure" immediately. If not, Guybrush will continue reading.

What's the right moment, you say? We don't know. There might be some kind of framerule or something where every 4 frames or so you get a frame where this works. But this is just speculation.