You can miss an item or two and still get 'One Toon's Trash'
So I missed buying the melted ice cream and decided to keep going, thinking there'd have to be at least an item or two leeway for the 20 items for this quest.
The quest where you get costumes for the splatters is key. You can get all 5 flower costumes for the one path as well as the four costums for Ian. That's 9 items total, plus the thank you note from the splatter for giving him the costume to initiate the quest at the lady's house, making 10 items just from this one quest alone. I still have two items in my inventory (the compass from the pirate and the thank you note from the splatter) and I just got the quest finished by using the 9 costumes.
Given the fact that I missed the melting ice cream, that means there's at least a 3-item leeway at this point, possibly more as I continue on.
Landed on exactly 500,000/500,000gs with exactly 18500 achievements and exactly 500 games. All 100% including DLC. Was going hard the first half of January to reach that goal! Finished it on 1-19-2016. Final achievement: Completion Serenity in Nero.