From Homestar Runner Wiki
"I think I left it in there a week or a month ago"
A Melt Shake is a drink sold in Free Country USA and according to Blubb-o's Commercial, Blubb-o's sells an Attempt-At-Competing-With-National-Coffee-Chains Premium Mocha-Chip Meltshake.
In road trip, Strong Bad finds a Melt Shake under the seat of the Gremlin. It later turns out to be Homestar's. He says he left it in there a week or a month ago.
Also, in hygiene, John has cups all over his room that look identical to a Melt Shake, but the logo isn't visible.