This puzzled me because I had never heard it… but it sort of sounded like a strange sexual euphemism from the way she said it. I investigated. The episode summary says this: A WHOOP pierces the air and Donna looks past Jody to the dance floor. Doug is doing the shopping cart with Sheriff Barbie. A cloud dims Donna’s sunshine. “I used to put the cans in his cart.” So… Doug is dancing with Barbie. He’s doing a dance called the “shopping cart”. In this dance, you dance like you are putting things in a shopping cart: https://m.youtube.com/watch?v=DW4Dg6fKzNU. H Donna looks over and sees Barbie putting imaginary cans in Doug’s imaginary cart as they dance. It reminds her of when she and Doug were a couple. She says she remembers when she used to be the one putting the cans in Doug’s cart, which she both means literally (she used to do this dance with Doug) and is using as euphemism for the other things she used to do with Doug (probably sexual). So this is not an English idiom - you couldn’t use it in another context and expect people to understand unless they were fans of the show. It’s more an example of how a sentence can have a particular meaning in a particular context and with a particular tone of voice.